[AMDGPU] Merge SIRemoveShortExecBranches into SIPreEmitPeephole

SIRemoveShortExecBranches is an optimisation so fits well in the
context of SIPreEmitPeephole.

Test changes relate to early termination from kills which have now
been lowered prior to considering branches for removal.
As these use s_cbranch the execz skips are now retained instead.
Currently either behaviour is valid as kill with EXEC=0 is a nop;
however, if early termination is used differently in future then
the new behaviour is the correct one.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D98917