[Utils] update_llc_test_checks.py: support AMDGPU backend: AMDGCN, r600 triples

Summary:
Lack of that support has taken me by surprise.
I need to add (or at least look at) some tests for https://reviews.llvm.org/D47980#1127615,
and i don't really fancy doing that by hand.

The asm pattern is quite similar to that of x86:
https://godbolt.org/g/hfgeds
just with `#` replaced with `;`

Reviewers: spatel, RKSimon, MaskRay, tstellar, arsenm

Reviewed By: arsenm

Subscribers: arsenm, kzhuravl, wdng, yaxunl, dstuttard, tpr, t-tye, rampitec, bogner, mareko, llvm-commits

Tags: #amdgpu

Differential Revision: https://reviews.llvm.org/D48001

llvm-svn: 334396