[RISCV][NFC] Parsed Immediates are Expressions (llvm#159888)

I find it very confusing that we have two different kinds of
"immediates":
- MCOperands in the backend that are `isImm()` which can only be numbers
- RISCVOperands in the parser that are `isImm()` which can contain
expressions

This change aims to make it clearer that in the AsmParser, we are
dealing with expressions, rather than just numbers.

Unfortunately, `isImm` comes from the `MCParsedAsmOperand`, which is
needed for Microsoft Inline Asm, so we cannot fully get rid of it.

Author: lenary