Was looking into supporting (srl (shl x, c1), c2) with c1 != c2 in dagcombiner,
this test changes, but makes update_llc_test_checks.py unhappy.
Many AMDGPU tests specify -march, not -mtriple, which results in update_llc_test_checks.py
defaulting to x86 asm function detection heuristics, which don't work here.
I propose to fix this by adding an infrastructure to map from -march to -mtriple,
in the UpdateTestChecks tooling.
Why this hardcoded list?