[globalisel][legalizer] G_LOAD/G_STORE NarrowScalar should not emit G_GEP x, 0.

Summary:
When legalizing G_LOAD/G_STORE using NarrowScalar, we should avoid emitting
	%0 = G_CONSTANT ty 0
	%1 = G_GEP %x, %0
since it's cheaper to not emit the redundant instructions than it is to fold them
away later.

Reviewers: qcolombet, t.p.northover, ab, rovka, aditya_nandakumar, kristof.beyls

Reviewed By: qcolombet

Subscribers: javed.absar, llvm-commits, igorb

Differential Revision: https://reviews.llvm.org/D32746

llvm-svn: 305340