[AArch64][GlobalISel] Fix the LegalityPredicate for lowerIf for G_LOAD/G_STORE

Summary:
Currently, Legalizer is trying to lower G_LOAD with a vector type
that has more than two elements due to the incorrect LegalityPredicate.

This patch fixes the issue by removing the multiplication by 8
as `MemDesc.Size` already contains the size in bits.

Reviewers: dsanders, aemerson

Reviewed By: dsanders

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

Differential Revision: https://reviews.llvm.org/D53679

llvm-svn: 345282