[GlobalISel] LegalizerHelper: Fix the incorrect alignment when splitting...

[GlobalISel] LegalizerHelper: Fix the incorrect alignment when splitting loads/stores in narrowScalar

Reviewers: dsanders, bogner, jpaquette, aemerson, ab, paquette

Reviewed By: dsanders

Subscribers: rovka, kristof.beyls, javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D53664

llvm-svn: 345292