[llvm] [AMDGPU] Improve isBasicBlockPrologue to only add necessary instructions (PR #113303)

[via llvm-commits]

alex-t wrote:

>  An instruction is part of prologue because it appears before the exec modification instruction.
I would not like to rely on the instructions' lexical order. An instruction belongs to the prologue if it is part of the def-use chain that ends up on the exec mask writing instruction.



https://github.com/llvm/llvm-project/pull/113303