[RISCV][CV-SIMD] Fix the immediate type in the shuffle pseudo expansion.

The immediate operand of the 'sci' variant of the shuffle instructions of the xcvsimd extension is always zero-extended. This commit fixes the expansion of the pseudo shuffle instruction by adding an unsigned immediate parameter (hence zero-extended) rather then a signed one as it was before.

This patch is a reboot of the previous PR #87

It is just a cleaner approach than a merge commit. The original change is good and copied untouched here.