[mips] Allowed dla instructions on 32-bit architectures.

Summary:
This is now the same as the behaviour of the GNU assembler. This was done
as it is required in order to build the Linux kernel with the integrated
assembler enabled.

Reviewers: dsanders, vkalintiris

Subscribers: dsanders, llvm-commits

Differential Revision: http://reviews.llvm.org/D13594

llvm-svn: 258400