[llvm] Split vgpr regalloc pipeline (PR #93526)

[Christudasan Devadasan via llvm-commits]

cdevadas wrote:

Added a negative test llvm/test/CodeGen/AMDGPU/wwm-regalloc-error.ll to capture the error during wwm-regalloc.
Fixed all tests earlier marked XFAIL.
Taken care of the review comments. 

https://github.com/llvm/llvm-project/pull/93526