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

Summary:
Note that this fix is very similar to what has already been
done for the masked load in https://reviews.llvm.org/D78608

Bugs:
https://bugs.llvm.org/show_bug.cgi?id=45563
https://bugs.llvm.org/show_bug.cgi?id=45833

Reviewers: craig.topper, nicolasvasilache, mehdi_amini

Reviewed By: craig.topper

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D79611