[X86] Fix a bug in getMemOpBaseRegImmOfs

Fix a crash in `getMemOpBaseRegImmOfs` that happens if the base of
`MemOp` is a frame index memory operand.  The fix is to have
`getMemOpBaseRegImmOfs` bail out in such cases.  We can possibly be more
clever here, if needed.

llvm-svn: 259456
diff --git a/llvm/lib/Target/X86/X86InstrInfo.cpp b/llvm/lib/Target/X86/X86InstrInfo.cpp
index 0c7ce7c..092e443 100644
--- a/llvm/lib/Target/X86/X86InstrInfo.cpp
+++ b/llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -4604,7 +4604,11 @@
 
   MemRefBegin += X86II::getOperandBias(Desc);
 
-  BaseReg = MemOp->getOperand(MemRefBegin + X86::AddrBaseReg).getReg();
+  MachineOperand &BaseMO = MemOp->getOperand(MemRefBegin + X86::AddrBaseReg);
+  if (!BaseMO.isReg()) // Can be an MO_FrameIndex
+    return false;
+
+  BaseReg = BaseMO.getReg();
   if (MemOp->getOperand(MemRefBegin + X86::AddrScaleAmt).getImm() != 1)
     return false;