[mips] Define certain instructions in microMIPS32r3

Instructions affected:
mthc1, mfhc1, add.d, sub.d, mul.d, div.d,
mov.d, neg.d, cvt.w.d, cvt.d.s, cvt.d.w, cvt.s.d

These instructions are now defined for
microMIPS32r3 + microMIPS32r6 in MicroMipsInstrFPU.td
since they shared their encoding with those already defined
in microMIPS32r6InstrInfo.td and have been therefore
removed from the latter file.

Some instructions present in MicroMipsInstrFPU.td which
did not have both AFGR64 and FGR64 variants defined have
been altered to do so.

Differential revision: https://reviews.llvm.org/D42738

llvm-svn: 324584