AMDGPU: Fix FP restore from being reordered with stack ops

In a function, s5 is used as the frame base SGPR. If a function
is calling another function, during the call sequence
it is copied to a preserved SGPR and restored.

Before it was possible for the scheduler to move stack operations
before the restore of s5, since there's nothing to associate
a frame index access with the restore.

Add an implicit use of s5 to the adjcallstack pseudo which ends
the call sequence to preven this from happening. I'm not 100%
satisfied with this solution, but I'm not sure what else would be
better.

llvm-svn: 328650