[mips] Rename the LA/LI/DLI TableGen definitions and classes. NFC.

Summary:
Use more reasonable names for these pseudo-instructions.
As there's only one definition tied to any one of these classes, I named them with abbreviated versions of their respective class' name.

Reviewers: dsanders

Reviewed By: dsanders

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D7831

llvm-svn: 231240