[mips][ias] Fix N32 and N64 .cprestore directive when inside .set noat region.

Summary:
r268058 unintentionally made the retrieval of the current assembler temporary
unconditional. This was fine for the existing tests but it broke the cases
where the assembler temporary is not needed (N32/N64 or not PIC) and is
unavailable due to a '.set noat' directive.

This fixes FreeBSD's libc.

Reviewers: emaste, sdardis, seanbruno

Subscribers: dsanders, emaste, sdardis, llvm-commits

Differential Revision: http://reviews.llvm.org/D20093

llvm-svn: 269179