[mips] Fix select patterns for MIPS64

When targetting MIPS64R6 some of the patterns for select were guarded by a
broken predicate. The predicate was supposed to test if a constant value
could fit in a 16 bit zero-extended field. Instead the value was tested to
fit in a 16 bit sign-extended field. For negative constants of native word
width this resulted in wrong code generation.

Reviewers: vkalintiris, dsanders

Differential Review: http://reviews.llvm.org/D19378

llvm-svn: 267151