[mips][ias] Fix O32 .cprestore directive when inside .set noat region and offset is in range.

Summary:
This expands on r269179 to fix an additional case that was not covered by our
tests. The assembler temporary is not needed when the .cprestore offset fits
inside a simm16 and it is not an error to use it inside a '.set noat' in this
case.

Reviewers: emaste, seanbruno, sdardis

Subscribers: dsanders, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20199

llvm-svn: 269295