[LoopVectorize] Pass a vector type to isLegalMaskedGather/Scatter

This is required to query the legality more precisely in the LoopVectorizer.

This adds another TTI function named 'forceScalarizeMaskedGather/Scatter'
function to work around the hack introduced for MVE, where
isLegalMaskedGather/Scatter would return an answer by second-guessing
where the function was called from, based on the Type passed in (vector
vs scalar). The new interface makes this explicit. It is also used by
X86 to check for vector widths where gather/scatters aren't profitable
(or don't exist) for certain subtargets.

Differential Revision: https://reviews.llvm.org/D115329