[mips] Fix `mtc1` and `mfc1` definitions for microMIPS R6

The `mtc1` and `mfc1` definitions in the MipsInstrFPU.td have MMRel,
but do not have StdMMR6Rel tags. When these instructions are emitted
for microMIPS R6 targets, `Mips::MipsR62MicroMipsR6` nor
`Mips::Std2MicroMipsR6` cannot find correct op-codes and as a result the
backend uses mips32 variant of the instructions encoding.

The patch fixes this problem by adding the StdMMR6Rel tag and check
instructions encoding in the test case.

Differential revision: https://reviews.llvm.org/D51482

llvm-svn: 341221