[RISCV] Simplify matchRegisterNameHelper interface. NFC

This previously returned a bool to indicate success or failure and
returns a register through an output parameter.

Some callers used the bool to check for success. Some callers checked
for RISCV::NoRegister.

To make everything uniform, return the MCRegister directly and update
all callers to use MCRegister::isValid().

Reviewed By: barannikov88

Differential Revision: https://reviews.llvm.org/D150049