As the FPR32 and FPR64 registers have the same names, use validateTargetOperandClass in RISCVAsmParser to coerce a parsed FPR32 to an FPR64 when necessary. The rest of this patch is very similar to the RV32F patch.
CCing @dylanmckay and @venkatra, as both AVR and Sparc backends do a similar sort of coercion in validateTargetOperandClass.