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

[via llvm-commits]

alex-t wrote:

> > An instruction belongs to the prologue if it is part of the def-use chain that ends up on the exec mask writing instruction.
> 
> Is this correct? like in the case below, the second sgpr-reload-from-vgpr is not in the prologue? v_readlane s0, v0, 1 v_readlane s1, v0, 2 s_or_b32 exec, exec, s0
> 
> > ```
Exactly. Only readlane producing s0 belongs to the prologue but another one, producing s1 does not.

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