[AMDGPU] NFC. Clean up check prefixes in fcmp test

Fix the test introduced in D136592 which appeared to have a few check lines
containing an un-checked prefix "GISEL-GFX".
Also canonicalize the other prefixes to minimize churn if SDag and GISel
diverge.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D147958