[llvm] [AMDGPU] Improve combining on v_ldexp (PR #190236)

[Jasmine Tang via llvm-commits]

badumbatish wrote:

> regbankcombine (remove combines before and after legalizer).
just to confirm you want me to just rip out this fmul combine from pre and post legalizer and just move it completely to regbankcombine?

ill update description of the PR soon after redo-ing this a bit more

https://github.com/llvm/llvm-project/pull/190236