[llvm] [AMDGPU][CodeGen] Place `S_NOP` after `S_SETREG_IMM32_B32` in predecessor MBB (PR #209620)

[Shilei Tian via llvm-commits]

shiltian wrote:

> When placing a `S_SET_VGPR_MSB` at the beginning of a block, check whether there exists a `S_SETREG_IMM32_B32(MODE)` in the predecessor block that falls-through into it; if there is one a `S_NOP` has to be inserted in between them.

What is the motivation of doing this?

https://github.com/llvm/llvm-project/pull/209620