[RISCV] Speed up RISCVRegisterInfo::needsFrameBaseReg when frame pointer isn't used. NFC (#89163)

The callee saved size is only used if there is a frame pointer. Sink the
code onto the frame pointer only path.

GitOrigin-RevId: 3e2aad412ada8f1db88c9e0f984a302deed74f4b
1 file changed