[SVE] Add support for folding for select + masked loads

Add folds to instcombine to support the removal of select instruction when the masked_load is guaranteed to zero the same lanes, i.e. select(mask, mload(,,mask,0), 0) -> mload(,,mask,0).

Patch originally authored by @paulwalker-arm

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D106376