[RISCV] Improve constant materialisation for stores of i8 negative constants (#92131)

This follows the same pattern as 20e62658. Although we
can't reduce the number of instructions used, if we are able to use a
sign-extended 6-bit immediate then the 16-bit c.li instruction can be
selected (thus saving code size). Although this _could_ be gated so it
only happens if C is enabled, I've opted not to because at worst it's
neutral and it doesn't seem helpful to add unnecessary divergence
between the RVC and non-RVC paths.