[mips] Fix the predicates of some aliases

Previously, some aliases were marked as not being available for microMIPS32R6,
but this was overridden at the top level.

Reviewers: atanasyan, abeserminji, smaksimovic

Differential Revision: https://reviews.llvm.org/D48321


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335053 91177308-0d34-0410-b5e6-96231b3b80d8