[mips] Remove two more redundant 'let Predicates = [HasStdEnc]' statements that were missed

Summary:
The InstSE class already initializes Predicates to [HasStdEnc].

No functional change (confirmed by diffing tablegen-erated files before and
after)

Differential Revision: http://reviews.llvm.org/D3548

llvm-svn: 207558