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

llvm-svn: 237133
diff --git a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
index 9c987c2..ceb7e15 100644
--- a/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
+++ b/llvm/lib/Target/R600/AMDGPUISelLowering.cpp
@@ -2677,6 +2677,7 @@
   NODE_NAME_CASE(BUILD_VERTICAL_VECTOR)
   NODE_NAME_CASE(CONST_DATA_PTR)
   case AMDGPUISD::FIRST_MEM_OPCODE_NUMBER: break;
+  NODE_NAME_CASE(SENDMSG)
   NODE_NAME_CASE(STORE_MSKOR)
   NODE_NAME_CASE(TBUFFER_STORE_FORMAT)
   case AMDGPUISD::LAST_AMDGPU_ISD_NUMBER: break;