[InstCombine] Pre-commit of test case showing miscompile bug in canEvaluateShuffled

Adding the reproducer from  https://bugs.llvm.org/show_bug.cgi?id=43689,
showing that instcombine is doing a bad transform. It transforms

  %0 = insertelement <2 x i16> undef, i16 %a, i32 0
  %1 = srem <2 x i16> %0, <i16 2, i16 1>
  %2 = shufflevector <2 x i16> %1, <2 x i16> undef, <2 x i32> <i32 undef, i32 0>

into

   %1 = insertelement <2 x i16> undef, i16 %a, i32 1
   %2 = srem <2 x i16> %1, <i16 undef, i16 2>

The undef denominator makes the whole srem undefined.

llvm-svn: 375207