[mips] Use MTHC1 when it is available (MIPS32r2 and later) for both FP32 and FP64

Summary:
To make this work for both AFGR64 and FGR64 register sets, I've had to make the
instruction definition consistent with the white lie (that it reads the lower
32-bits of the register) when they are generated by expandBuildPairF64().

Corrected the definition of hasMips32r2() and hasMips64r2() to include
MIPS32r6 and MIPS64r6.

Depends on D3956

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D3957