Do not mark non-phi loop var def as loop-carried dep

[thopre]

Summary

As spotted by Sumanth Gundapaneni, commit
c8be35293c82f216e975c56f62ddf9199a22f2e2 incorrectly marked loop
variable not defined by a PHI as loop-carried dependencies. This commit
fixes that.

[bcahoon]

llvm/lib/CodeGen/MachinePipeliner.cpp

2283

If we don't know the definition for BaseOpsS or BaseOpsD, then we have to be conservative and assume a loop carried dependence.

Perhaps we need to handle two cases, one when BaseOpsS is the same as BaseOpsD, and the other is when they aren't the same.

llvm/lib/CodeGen/MachinePipeliner.cpp

if (!TII->getMemOperandWithOffset(*SI, BaseOpS, OffsetS, OffsetSIsScalable,
     return true;
 
   assert(!OffsetSIsScalable && !OffsetDIsScalable &&
          "Expected offsets to be byte offsets");
 
   MachineInstr *DefS = MRI.getVRegDef(BaseOpS->getReg());
   MachineInstr *DefD = MRI.getVRegDef(BaseOpD->getReg());
   if (!DefS || !DefD || !DefS->isPHI() || !DefD->isPHI())
-    return true;
+    return false;
 
   unsigned InitValS = 0;
   unsigned LoopValS = 0;
   unsigned InitValD = 0;
   unsigned LoopValD = 0;
   getPhiRegs(*DefS, BB, InitValS, LoopValS);
   getPhiRegs(*DefD, BB, InitValD, LoopValD);
   MachineInstr *InitDefS = MRI.getVRegDef(InitValS);