[RISCV] Merge RISCVMCInstLower.cpp into RISCVAsmPrinter.cpp.

The separation here doesn't make much sense. I think it's a
leftover from the creation of the MC layer that has been
replicated to new targets.

By merging them we can avoid passing the AsmPrinter to the
MCInstLowering functions. We can make them member functions instead.

I think we can still do more integration of lowerSymbolOperand
and lowerRISCVVMachineInstrToMCInst, but I wanted to get feedback
on the direction first.

Reviewed By: asb, barannikov88

Differential Revision: https://reviews.llvm.org/D152311