[UpdateTestChecks] Fix PowerPC RE to support AIX assembly

Current update_llc_test_checks.py cannot generate checks for AIX
(powerpc64-ibm-aix-xcoff) properly. Assembly generated is little bit
different from Linux. So I use begin function comment here to capture
function name.

Reviewed By: MaskRay, steven.zhang

Differential Revision: https://reviews.llvm.org/D93676