[AArch64][GlobalISel] Select immediate modes for ADD when selecting G_GEP

Before, we weren't able to select things like this for G_GEP:

add	x0, x8, #8

And instead we'd materialize the 8.

This teaches GISel to do that. It gives some considerable code size savings
on 252.eon-- about 4%!

Differential Revision: https://reviews.llvm.org/D65248

llvm-svn: 366959