[X86][GlobalISel] Add minimal call lowering support to the IRTranslator

Summary:
    Add basic functionality to support call lowering for X86.
    Currently only supports functions which return void and take zero arguments.
    Inspired by commit 286573.

Reviewers: ab, qcolombet, t.p.northover

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D26593

llvm-svn: 286935