[mips] Stop caching the result of hasMips64(), isABI_O32(), isABI_N32(), and isABI_N64() from MipsSubTarget in MipsTargetLowering

Summary:
The short name is quite convenient so provide an accessor for them instead.

No functional change

Depends on D3177

Reviewers: matheusalmeida

Reviewed By: matheusalmeida

Differential Revision: http://llvm-reviews.chandlerc.com/D3178