[globalisel] Allow SrcOp to convert an APInt and render it as an immediate...

[globalisel] Allow SrcOp to convert an APInt and render it as an immediate operand (MO.isImm() == true)

Summary:
This is tested by D61289 but has been pulled into a separate patch at
a reviewers request.

Reviewers: bogner, aditya_nandakumar, volkan, aemerson, paquette, arsenm, rovka

Reviewed By: arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D61321

llvm-svn: 368063