Recommit "[RISCV] Remove include of RISCVRegisterInfo.h from RISCVBaseInfo.h. NFCI"

This reverts 78191725 and recommits
78191725.

I've changed getRegForInlineAsmConstraint to not use a std::pair
of Register in a previous commit. Hopefully that fixes the reported
issue with expensive checks on Windows. I'm still not sure exactly
why this commit removing an include affected a different file.

Original message:

RISCVRegisterInfo.h is part of the CodeGen layer. The Utils library
is intended to be shared with the MC layer so shouldn't use files
from the CodeGen layer.

The register enum names are already available from
RISCVMCTargetDesc.h. It appears what was coming from this include
was a transitive include of the Register class which I've replaced
with MCRegister. Register has a constructor from MCRegister so it
should be convertible.