[mips] Add regression tests for parenthetic expressions in MIPS assembly.

Summary:
These expressions already worked but weren't tested.

Patch by Robert N. M. Watson and David Chisnall (it was originally two patches)
Their work was sponsored by: DARPA, AFRL

Differential Revision: http://llvm-reviews.chandlerc.com/D3156

llvm-svn: 204612