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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375448 91177308-0d34-0410-b5e6-96231b3b80d8