[mips] Optimize materialization of i64 constants

Avoid MipsAnalyzeImmediate usage if the constant fits in an 32-bit
integer. This allows us to generate the same instructions for the
materialization of the same constants regardless the width of their
type.

Patch by: Vasileios Kalintiris

Contributions by: Simon Dardis

Reviewers: Daniel Sanders

Differential Review: https://reviews.llvm.org/D21689

llvm-svn: 276628