AMDGPU: VALU carry-in and v_cndmask condition cannot be EXEC

The hardware will only forward EXEC_LO; the high 32 bits will be zero.

Additionally, inline constants do not work. At least,

   v_addc_u32_e64 v0, vcc, v0, v1, -1

which could conceivably be used to combine (v0 + v1 + 1) into a single
instruction, acts as if all carry-in bits are zero.

The llvm.amdgcn.ps.live test is adjusted; it would be nice to combine

   s_mov_b64 s[0:1], exec
   v_cndmask_b32_e64 v0, v1, v2, s[0:1]

into

   v_mov_b32 v0, v3

but it's not particularly high priority.

Fixes dEQP-GLES31.functional.shaders.helper_invocation.value.*

llvm-svn: 314522