[mips] Sink PredicateControl further down the class hierarchy.

Previously PredicateControl in some cases was a member of <X>Inst classes
for some X (DSP, EVA) or was in more irregular place in the hierarchry
for any given instruction.

This patch moves PredicateControl down to the root so that it is consistently
available. Then correct the base class of microMIPS instructions as using
EncodingPredicates instead of the general Predicates field of Instruction.

Reviewers: smaksimovic, abeserminji, atanasyan

Differential Revision: https://reviews.llvm.org/D47526

llvm-svn: 333536