[Fix][Relax] Scope planned storage to if branches

[javierdejesusda]

Fixes #20191.

StaticPlanBlockMemory plans with one token pool per function, and its rewriter emits R.memory.alloc_storage only at the first use of a token. A token released in one if branch can be reused in the other branch or after the if, where that storage variable is not defined. The planned module is then ill-formed, and the VM segfaults for cond=False in the issue script.

The rewriter now restores its token-to-storage-variable map when it leaves a SeqExpr, so a token first used in a branch gets a new alloc_storage wherever it is reused outside that branch. Storage allocated before the if is still shared with the branches.

Tests:

• python -m pytest -q tests/python/relax/test_transform_static_plan_block_memory.py (32 passed). Two of the three new tests fail without the change; the third pins that storage allocated before the if stays shared, and passes either way.
• The issue script without exec_mode="bytecode" (relax.build no longer takes it), built with the default pipeline for llvm, returns x for cond=True and cond=False; before the change cond=False segfaults.
• pre-commit run --files src/relax/transform/static_plan_block_memory.cc tests/python/relax/test_transform_static_plan_block_memory.py