[llvm] [AMDGPU][GIsel][NPM] Port AMDGPUPostLegalizerCombiner to NPM (PR #216274)

[Keshav Vinayak Jha via llvm-commits]

keshavvinayak01 wrote:

@arsenm  I confirmed that the legacy pass currently passes nullptr for CSEInfo. I tried enabling CSE for both the legacy and new pass managers, but this caused CI issues.

Fixing those observer issues and updating the resulting MIR checks is broader than the NPM port. I'll split up and create a pre-requisite PR for that change first before landing this.

https://github.com/llvm/llvm-project/pull/216274