Check for empty YMM use-def lists in X86VZeroUpper.

The previous MRI.isPhysRegUsed(YMM0) would also return true when the
function contains a call to a function that may clobber YMM0. That's
most of them.

Checking the use-def chains allows us to skip functions that don't
explicitly mention YMM registers.

llvm-svn: 166110

llvm/lib/Target/X86/X86VZeroUpper.cpp

@@ -147,7 +147,7 @@ bool VZeroUpperInserter::runOnMachineFunction(MachineFunction &MF) {
   const TargetRegisterClass *RC = &X86::VR256RegClass;
   for (TargetRegisterClass::iterator i = RC->begin(), e = RC->end();
        i != e; i++) {
-    if (MRI.isPhysRegUsed(*i)) {
+    if (!MRI.reg_nodbg_empty(*i)) {
       YMMUsed = true;
       break;
     }