[MIPS GlobalISel] Select instructions to load and store i32 on stack

Add code for selection of G_LOAD, G_STORE, G_GEP, G_FRAMEINDEX and
G_CONSTANT. Support loads and stores of i32 values.

Patch by Petar Avramovic.

Differential Revision: https://reviews.llvm.org/D48957

llvm-svn: 337168