[llvm] [AMDGPU][CodeGen] Don't always create `S_SET_VGPR_MSB` after `S_SETREG_B32` (PR #210042)

[Lucas Ramirez via llvm-commits]

lucas-rami wrote:

> Jus curious: how often do we encounter this case in real world code?

Not sure honestly. I am making more meaningful changes to this pass and noticing these tiny inefficiencies so I am addressing them as I go. I have got another peephole optimization coming up for the complementary case (`size<=12||offset!=0`).

https://github.com/llvm/llvm-project/pull/210042