[llvm] [CodeGen] Fixed vector halving bug for masked load

Summary:
Given a VL=14 that is enveloped by a proper VL=16, splitting the
masked load using the enveloping halving VL=8/8 should yields
should eventually yield V=8/5. This fixes various assert failures
in getHalfNumVectorElementsVT() and IncrementMemoryAddress().

Note, I suspect similar fixes will be needed for other masked
operations, but for now I send out a fix for masked load only.

Bugzilla issue 45563
https://bugs.llvm.org/show_bug.cgi?id=45563

Reviewers: craig.topper, mehdi_amini, nicolasvasilache

Reviewed By: craig.topper

Subscribers: hiraditya, dmgreen, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D78608