AMDGPU/InsertWaitcnts: Update VGPR/SGPR bounds when brackets are merged

Summary:
Fix an issue where VGPR/SGPR bounds are not properly extended when brackets are merged.
This manifests as missing waitcnt insertions when multiple brackets are forwarded to a successor block and the first forward has lower VGPR/SGPR bounds.

Irreducible loop test has been extended based on a CTS failure detected for GFX9.

Reviewers: nhaehnle

Reviewed By: nhaehnle

Subscribers: arsenm, kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, jfb, llvm-commits

Differential Revision: https://reviews.llvm.org/D55602

llvm-svn: 349611