[AMDGPU] Add implicit negative check for the set_gpr_idx tests

The only effect of the optimization is to remove s_set_gpr_idx_*
instructions, and update_mir_test_checks.py always inserts CHECK: rather
than CHECK-NEXT: checks, so without this implicit negative check, the
tests would always pass even if the optimization did nothing.

Differential Revision: https://reviews.llvm.org/D101622