Changes

AMDGPU: Slightly restructure m0 init code
This will allow using another operation to produce the glue in a future
change.
llvm-svn: 375447

AMDGPU: Stop adding m0 implicit def to SGPR spills
r375293 removed the SGPR spilling with scalar stores path, so this is no
longer necessary. This also always had the defect of adding the def even
when this path wasn't in use.
llvm-svn: 375448