[mips] sgtu, s[rl]l, sra, dnegu, neg instruction aliases

Add the instruction alias sgtu (register form only), two operand forms of
s[rl]l and sra, and missing single/two operand forms of dnegu/neg.

Reviewers: dsanders

Differential Revision: https://reviews.llvm.org/D22752

llvm-svn: 276736