[RISCV] Simplify interface of RISCVAsmPrinter::lowerToMCInst [nfc] (#156482)
The only case which returns true is just pypassing this routine for
custom logic. Given the caller *already* has to special case this to
even fall into this routine, let's just put the logic in one place.
Note that the code had a guard for a malformed attribute which is
unreachable, and was converted into an assert. The verifier enforces
that the function attribute is well formed if present.