AMDGPU: Reduce number of copies emitted

AMDGPU: Reduce number of copies emitted

Instead of always inserting a copy in case
the super register is itself a subregister,
only extract to the super reg class if this is
actually the case.

This shouldn't really change codegen, but
makes looking at the output of SIFixSGPRCopies
easier to read.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@248467 91177308-0d34-0410-b5e6-96231b3b80d8