[RISCV] Rename RISCVISD::SHFLI to RISCVISD::SHFL and don't require the second...

[RISCV] Rename RISCVISD::SHFLI to RISCVISD::SHFL and don't require the second operand to be an immediate.

Prep work for adding intrinsics in the future.

Left an assert that the input is constant in ReplaceNodeResults,
as the intrinsic shouldn't go through that path.