[X86] lowerShuffleAsElementInsertion - fold to...

[X86] lowerShuffleAsElementInsertion - fold to or(vzext_movl(scalar_to_vector(zext(x))), and(constant, mask))

The logic in this function is a bit of a mess, but masking a vector constant should allow us to OR the zero-extended i8/i16 scalar value in place.

We can do more here - reusing the OR pattern if the relevant unused elements are known zero etc. but this is enough to address a regression from D127115.