[mips] Access $at only if necessary.

Summary:
This patch doesn't really change the logic behind expandMemInst but it allows
us to assemble .S files that use .set noat with some macros. For example:

.set noat
lw $k0, offset($k1)

Can expand to:
lui	$k0, %hi(offset)
addu	$k0, $k0, $k1
lw	$k0, %lo(offset)($k0)

with no need to access $at.

Reviewers: dsanders, vmedic

Reviewed By: dsanders, vmedic

Differential Revision: http://reviews.llvm.org/D4159