AMDGPU: Expand shuffle testing with generated tests (llvm#123574)

Add some generated tests with every shuffle permutation
for relevant vector element types and sizes. Not sure if this
is going overboard with the number of tests. I pruned out the largest
cases (16 and 32-bit cases are impractically large), and there's
redundancy when testing the pointer cases (at least for SelectionDAG).

This uses inline assembly to produce sample values because of how the
ABI is lowered when using a function argument. Since we break all
arguments into 32-bit pieces, a shuffle never ends up forming. We
need separate handling to reconstruct shuffles in contexts involving
physical registers in ABI contexts.

I wrote a small tool to generate these, so I can easily change the
exact test body. Not sure if it's worth posting anywhere.

This is in preparation for making better use of v_pk_mov_b32,
v_mov_b64 and s_mov_b64 in shuffles.

Author: arsenm