[MIPS] For vectors, select `add %x, C` as `sub %x, -C` if it results in inline immediate

Summary:
As discussed in https://reviews.llvm.org/D62341#1515637,
for MIPS `add %x, -1` isn't optimal. Unlike X86 there
are no fastpaths to matearialize such `-1`/`1` vector constants,
and `sub %x, 1` results in better codegen,
so undo canonicalization

Reviewers: atanasyan, Petar.Avramovic, RKSimon

Reviewed By: atanasyan

Subscribers: sdardis, arichardson, hiraditya, jrtc27, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66805

llvm-svn: 372254