[mips] [IAS] Add support for the DLA pseudo-instruction and fix problems with DLI

Summary: It is the same as LA, except that it can also load 64-bit addresses and it only works on 64-bit MIPS architectures.

Reviewers: tomatabacu, seanbruno, vkalintiris

Subscribers: brooks, seanbruno, emaste, llvm-commits

Differential Revision: http://reviews.llvm.org/D9524

llvm-svn: 245208