[mips] Use multiclass patterns for f32/f64 comparisons and i32 selects.

Summary:
Although the multiclass for i32 selects might seem redundant as it has
only one instantiation, we will use it to replace the correspondent
patterns in Mips64r6InstrInfo.td in follow-up commits.

Reviewers: dsanders

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D14612

llvm-svn: 255110