ARM: Fix more fast-isel verifier failures.

Teach the generic instruction selection helper functions to constrain
the register classes of their input operands. For non-physical register
references, the generic code needs to be careful not to mess that up
when replacing references to result registers. As the comment indicates
for MachineRegisterInfo::replaceRegWith(), it's important to call
constrainRegClass() first.

rdar://12594152

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@188593 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

@@ -497,6 +497,10 @@ bool SelectionDAGISel::runOnMachineFunction(MachineFunction &mf) {
       if (J == E) break;
       To = J->second;
     }
+    // Make sure the new register has a sufficiently constrained register class.
+    if (TargetRegisterInfo::isVirtualRegister(From) &&
+        TargetRegisterInfo::isVirtualRegister(To))
+      MRI.constrainRegClass(To, MRI.getRegClass(From));
     // Replace it.
     MRI.replaceRegWith(From, To);
   }