[LV] Use available masked vector function variants when required

[LV] Use available masked vector function variants when required

LLVM has the ability to vectorize using function variants that require
a mask by creating an all-true mask, and to vectorize a conditional
call via scalarization, now we want to join the two parts together
and use a masked variant when a mask is required.

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D136251