[mips] Correct c.cond.fmt instruction definition.

Permit explicit $fcc<X> operand in c.cond.fmt instruction.

Add c.cond.fmt to the MIPS to microMIPS instruction mapping table.

Check that $fcc1 - $fcc7 are unusable for MIPS-I to MIPS-III for
c.cond.fmt, bc1t, bc1f.

Reviewers: seanbruno, zoran.jovanovic, vkalintiris

Differential Revision: https://reviews.llvm.org/D24510

llvm-svn: 292117