[X86] Return UNDEF from LowerScalarImmediateShift when the shift amount is out of...

[X86] Return UNDEF from LowerScalarImmediateShift when the shift amount is out of range.

I think we only turn out of range shiftss to undef when
all elements are out of range or the shift amount is a splat out
of range. I'm not sure which, I didn't check.

During lowering we can split a shift where some elements
are out of range into multiple shifts. This can create a
new shift with a splat shift amount that is out of range.

This patch returns undef for this case.

Fixes PR42615.

Differential Revision: https://reviews.llvm.org/D64699

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@366096 91177308-0d34-0410-b5e6-96231b3b80d8