UpdateTestChecks: fix AMDGPU handling

Summary:
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.

Reviewers: RKSimon, MaskRay, arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D62099

llvm-svn: 361101