[mips] Support the "o" inline asm constraint

As well as other LLVM targets we do not handle "offsettable"
memory addresses in any special way. In other words, the "o" constraint
is an exact equivalent of the "m" one. But some existing code require
the "o" constraint support.

This fixes PR42589.

Differential Revision: https://reviews.llvm.org/D64792

llvm-svn: 366299