[llvm] [AMDGPU] Handle `s_setreg_imm32_b32` targeting `MODE` register (PR #174681)

[Shilei Tian via llvm-commits]

shiltian wrote:

> Check `MostRecentModeSet`. Pass can update previously inserted `S_SET_VGPR_MSB` with a new mask, in which case setreg after it also needs to be updated.

ACK. Will take a look.

https://github.com/llvm/llvm-project/pull/174681