feat: remove max_bit_size from Instruction::Truncate

[TomAFrench]

Description

Problem*

Resolves #6998

Summary*

This PR just tries to remove this field as the only place it impacts ACIR seems irrelevant.

Additional Context

Documentation*

Check one:

• No documentation needed.
• Documentation included in this PR.
• [For Experimental Features] Documentation to be submitted in a separate PR.

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[github-actions]

Changes to circuit sizes

Generated at commit: 599b3503c8b98f203f1b7622524d0feee662702e, compared to commit: a0ffedfdf445b2dc6655cafa6fe614958ddb9603

🧾 Summary (10% most significant diffs)

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
signed_cmp	+10 ❌	+15.87%	+2,763 ❌	+1196.10%
signed_comparison	+20 ❌	+13.99%	+2,790 ❌	+936.24%
signed_arithmetic	+50 ❌	+27.17%	+834 ❌	+28.45%
unary_operator_overloading	+5 ❌	+35.71%	+710 ❌	+25.62%
regression_2660	+5 ❌	+27.78%	+710 ❌	+25.59%

---

Full diff report 👇

Program	ACIR opcodes (+/-)	%	Circuit size (+/-)	%
signed_cmp	73 (+10)	+15.87%	2,994 (+2,763)	+1196.10%
signed_comparison	163 (+20)	+13.99%	3,088 (+2,790)	+936.24%
signed_arithmetic	234 (+50)	+27.17%	3,765 (+834)	+28.45%
unary_operator_overloading	19 (+5)	+35.71%	3,481 (+710)	+25.62%
regression_2660	23 (+5)	+27.78%	3,485 (+710)	+25.59%
simple_shift_left_right	19 (+5)	+35.71%	3,528 (+710)	+25.20%
sha2_byte	18,664 (+3,435)	+22.56%	90,566 (+8,602)	+10.49%
hashmap	38,925 (+2,025)	+5.49%	105,210 (+6,630)	+6.73%
sha256_var_witness_const_regression	1,180 (+105)	+9.77%	17,466 (+989)	+6.00%
conditional_1	2,875 (+15)	+0.52%	10,780 (+491)	+4.77%
6	376 (+30)	+8.67%	11,648 (+530)	+4.77%
conditional_regression_short_circuit	397 (+30)	+8.17%	11,669 (+530)	+4.76%
sha256	1,571 (+135)	+9.40%	25,651 (+1,068)	+4.34%
bit_shifts_runtime	344 (+60)	+21.13%	4,108 (+163)	+4.13%
u16_support	74 (+5)	+7.25%	2,995 (+103)	+3.56%
bit_shifts_comptime	23 (+10)	+76.92%	2,934 (+90)	+3.16%
sha256_var_size_regression	10,537 (+345)	+3.39%	64,567 (+1,604)	+2.55%
bench_sha256	204 (+20)	+10.87%	7,193 (+144)	+2.04%
array_dynamic_blackbox_input	1,382 (+20)	+1.47%	21,805 (+423)	+1.98%
regression	322 (+30)	+10.27%	3,889 (+62)	+1.62%
array_dynamic_nested_blackbox_input	250 (+5)	+2.04%	7,406 (+102)	+1.40%
bench_sha256_30	5,859 (+600)	+11.41%	126,180 (+1,679)	+1.35%
bench_sha256_100	19,509 (+2,000)	+11.42%	413,388 (+5,389)	+1.32%
signed_division	210 (+10)	+5.00%	3,702 (+27)	+0.73%
sha256_regression	22,532 (+190)	+0.85%	181,583 (+1,197)	+0.66%
5_over	25 (+5)	+25.00%	3,508 (+20)	+0.57%
regression_4449	8,897 (+350)	+4.10%	286,170 (+1,306)	+0.46%
ecdsa_secp256k1	510 (+20)	+4.08%	43,704 (+143)	+0.33%
sha256_var_padding_regression	3,848 (+30)	+0.79%	194,096 (+443)	+0.23%
ram_blowup_regression	140,761 (+5)	+0.00%	643,632 (+373)	+0.06%
binary_operator_overloading	127 (+5)	+4.10%	4,190 (-72)	-1.69%
regression_struct_array_conditional	71 (+5)	+7.58%	3,124 (-72)	-2.25%
u128	471 (+5)	+1.07%	4,263 (-162)	-3.66%

[github-actions]

Compilation Report

Program	Compilation Time	%
sha256_regression	1.090s	9%
regression_4709	0.822s	-2%
ram_blowup_regression	16.400s	1%
rollup-root	3.574s	8%
rollup-merge	1.820s	1%
rollup-block-merge	3.330s	-2%
rollup-base-public	31.160s	-2%
rollup-base-private	10.760s	1%
private-kernel-tail	0.965s	-3%
private-kernel-reset	6.424s	2%
private-kernel-inner	1.984s	0%

[github-actions]

Execution Report

Program	Execution Time	%
sha256_regression	0.053s	1%
regression_4709	0.001s	0%
ram_blowup_regression	0.609s	0%
rollup-root	0.105s	0%
rollup-merge	0.006s	0%
rollup-block-merge	0.104s	0%
rollup-base-public	1.220s	-1%
rollup-base-private	0.449s	-1%
private-kernel-tail	0.019s	0%
private-kernel-reset	0.313s	-1%
private-kernel-inner	0.068s	0%

[github-actions]

Compilation Memory Report

Program	Peak Memory
keccak256	77.72M
workspace	123.47M
regression_4709	424.09M
ram_blowup_regression	1.58G
rollup-base-public	2.51G
rollup-base-private	901.14M
private-kernel-tail	207.18M
private-kernel-reset	669.26M
private-kernel-inner	294.40M

[github-actions]

Execution Memory Report

Program	Peak Memory
keccak256	74.67M
workspace	123.46M
regression_4709	316.00M
ram_blowup_regression	512.61M
rollup-base-public	478.93M
rollup-base-private	325.52M
private-kernel-tail	180.47M
private-kernel-reset	245.16M
private-kernel-inner	208.55M