[AMDGPU] Skip CFIInstructions in SIInsertWaitcnts

Summary:
CFI emitted during PEI at the beginning of the prologue needs to apply
to any inserted waitcnts on function entry.

Reviewers: arsenm, t-tye, RamNalamothu

Reviewed By: arsenm

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

Tags: #llvm, #debug-info

Differential Revision: https://reviews.llvm.org/D76881