Fix PR12821.

RAFast must add an <imp-def> operand when it is rewriting a sub-register
def that isn't a read-modify-write.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156777 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/RegAllocFast.cpp

@@ -674,6 +674,12 @@ bool RAFast::setPhysReg(MachineInstr *MI, unsigned OpNum, unsigned PhysReg) {
     MI->addRegisterKilled(PhysReg, TRI, true);
     return true;
   }
+
+  // A <def,read-undef> of a sub-register requires an implicit def of the full
+  // register.
+  if (MO.isDef() && MO.isUndef())
+    MI->addRegisterDefined(PhysReg, TRI);
+
   return MO.isDead();
 }