MIPS32R6: Fix floating point compares in mterp.

This fixes failures in the following tests:
003-omnibus-opcodes
082-inline-execute
083-compiler-regressions
107-int-math2
422-type-conversion
570-checker-select

Test: 'run-test --interpreter <tests above>'
      in QEMU for MIPS64R6/MIPS32R6

Change-Id: I11b4f1df7c212932c92d41fbc98c00465d03e850