AMDGPU: Fix verifier error when spilling SGPRs

The current SGPR spilling test does not stress this
because it is using s_buffer_load instructions to
increase SGPR pressure and spill, but their output
operands have the same SReg_32_XM0 constraint. This fixes
an error when the SReg_32 output from most instructions
is spilled.

llvm-svn: 270301