[MIPS GlobalISel] Add lowerCall

Add minimal support to lower function calls.
Support only functions with arguments/return that go through registers
and have type i32.

Patch by Petar Avramovic.

Differential Revision: https://reviews.llvm.org/D45627


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@334071 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed