[X86] isTargetShuffleEquivalent - assert the expected mask is correctly formed. NFCI.

While we don't make any assumptions about the actual mask, assert that the expected mask only contains valid mask element values.

llvm-svn: 366066

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -10001,6 +10001,8 @@ static bool isTargetShuffleEquivalent(ArrayRef<int> Mask,
   int Size = Mask.size();
   if (Size != (int)ExpectedMask.size())
     return false;
+  assert(isUndefOrZeroOrInRange(ExpectedMask, 0, 2 * Size) &&
+         "Illegal target shuffle mask");
 
   for (int i = 0; i < Size; ++i)
     if (Mask[i] == SM_SentinelUndef)