[GlobalISel] IRTranslator: Translate the intrinsics ignored by CodeGen

Summary:
Translate `llvm.assume`, `llvm.var.annotation` and `llvm.sideeffect` to nothing
as they have no effect on CodeGen.

Reviewers: qcolombet, aditya_nandakumar, dsanders, paquette, aemerson, arsenm

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63022

llvm-svn: 362834