AMDGPU: Fix SDWA crash on inline asm

This was only searching for explicit defs,
and asserting for any implicit or variadic
instruction defs, like inline asm.

llvm-svn: 319826
diff --git a/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp b/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
index 59bc44b..5fda45f 100644
--- a/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
+++ b/llvm/lib/Target/AMDGPU/SIPeepholeSDWA.cpp
@@ -324,7 +324,8 @@
       return &DefMO;
   }
 
-  llvm_unreachable("invalid reg");
+  // Ignore implicit defs.
+  return nullptr;
 }
 
 uint64_t SDWASrcOperand::getSrcMods(const SIInstrInfo *TII,