[MI] Fix MachineInstr::isInvariantLoad.

[jlebar]

Summary

Previously it would say we had an invariant load if any of the memory
operands were invariant. But the load should be invariant only if *all*
the memory operands are invariant.

No testcase because this has proven to be very difficult to tickle in
practice. As just one example, ARM's ldrd instruction, which loads 64
bits into two 32-bit regs, is theoretically affected by this. But when
it's produced, it loses its memoperands' invariance bits!

[hfinkel]

LGTM

[jfb]

lgtm after the facts. ty!

llvm/trunk/lib/CodeGen/MachineInstr.cpp

if (memoperands_empty())
     return false;
 
   const MachineFrameInfo *MFI = getParent()->getParent()->getFrameInfo();
 
   for (mmo_iterator I = memoperands_begin(),
        E = memoperands_end(); I != E; ++I) {
     if ((*I)->isVolatile()) return false;
     if ((*I)->isStore()) return false;
-    if ((*I)->isInvariant()) return true;
-
+    if ((*I)->isInvariant()) continue;
 
     // A load from a constant PseudoSourceValue is invariant.
     if (const PseudoSourceValue *PSV = (*I)->getPseudoValue())
       if (PSV->isConstant(MFI))
         continue;
 
     if (const Value *V = (*I)->getValue()) {
       // If we have an AliasAnalysis, ask it whether the memory is constant.