[AMDGPU] Rename SIInsertSkips Pass

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