[GlobalISel] CSEMIRBuilder: Add support for G_GEP

Summary:
This patch adds G_GEP to `shouldCSEOpc` so that it can be CSEd. It also refactors
`translateGetElementPtr` by replacing `createGenericVirtualRegister` calls with types.

Reviewers: aditya_nandakumar, arsenm, dsanders, paquette, aemerson

Reviewed By: aditya_nandakumar

Subscribers: wdng, rovka, javed.absar, hiraditya, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66316

llvm-svn: 369070