[mips] Properly select abs and sqrt instructions

- Alter abs for micromips to have both AFGR64 and FGR64
  variants, same as sqrt
- Remove sqrt and abs from MicroMips32r6InstrInfo.td,
  use micromips FGR64 variants
- Restrict non-micromips abs/sqrt with NotInMicroMips
  predicate

Differential revision: https://reviews.llvm.org/D41439

llvm-svn: 323184