[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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@211165
91177308-0d34-0410-b5e6-
96231b3b80d8