[mips] Expose adjustStackPtr() from MipsInstrInfo. NFC.

Summary:
adjustStackPtr() is implemented from both MipsSEInstrInfo and
Mips16InstrInfo. It makes sense to expose this function from
MipsInstrInfo and avoid explicit casting in some cases.

Depends on D8638.

Reviewers: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D8640

llvm-svn: 233905