R600/SI: Remove explicit m0 operand from s_sendmsg
Instead add m0 as an implicit operand. This allows us to avoid using
the M0Reg register class and eliminates a number of unnecessary spills
when using s_sendmsg instructions. This impacts one shader in the
shader-db:
SGPRS: 48 -> 40 (-16.67 %)
VGPRS: 112 -> 108 (-3.57 %)
Code Size: 40132 -> 38796 (-3.33 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Scratch: 2048 -> 0 (-100.00 %) bytes per wave
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@237133
91177308-0d34-0410-b5e6-
96231b3b80d8