[mips][ias] Handle more complicated expressions for memory operands

This patch teaches ias for mips to handle expressions such as
(8*4)+(8*31)($sp). Such expression typically occur from the expansion
of multiple macro definitions.

This partially resolves PR/30383.

Thanks to Sean Bruno for reporting the issue!

Reviewers: zoran.jovanovic, vkalintiris

Differential Revision: https://reviews.llvm.org/D24667

llvm-svn: 284485