Disable null export (for kills) when a frontend defines a pixel
shader as not exporting using amdgpu-color-export and
amdgpu-depth-export function attrbutes.
This allows the generation of export free pixel shaders.
Reviewed By: foad
Differential Revision: https://reviews.llvm.org/D105683
Pass no longer handles skips. Pass now removes unnecessary
unconditional branches and lowers early termination branches.
Hence rename to SILateBranchLowering.
Move code to handle returns to epilog from SIPreEmitPeephole
into SILateBranchLowering. This means SIPreEmitPeephole only
contains optional optimisations, and all required transforms
are in SILateBranchLowering.
Reviewed By: arsenm
Differential Revision: https://reviews.llvm.org/D98915
