AMDGPU: Allow a SGPR for the conditional KILL operand

Patch by: Bas Nieuwenhuizen

Just use the _e64 variant if needed. This should be possible as per

def : Pat <
  (int_amdgcn_kill (i1 (setcc f32:$src, InlineFPImm<f32>:$imm, cond:$cond))),
  (SI_KILL_F32_COND_IMM_PSEUDO $src, (bitcast_fpimm_to_i32 $imm), (cond_as_i32imm $cond))
> ;

I don't think we can get an immediate for the other operand for which we
need the second 32-bit word.

https://reviews.llvm.org/D42302

llvm-svn: 323706