[MIPS GlobalISel] Legalize i8 and i16 add

Legalize G_ADD for types smaller than i32.
LegalizationArtifactCombiner replaces extend instructions with appropriate
bitwise instructions.

Patch by Petar Avramovic.

Differential Revision: https://reviews.llvm.org/D51213


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@340697 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed