[RISCV][InsertVSETVLI] Rewrite scalar insert forward rule in terms of demanded fields

This is mostly geared at consolidating logic into one form to reduce code duplication, but also has the effect of being a slight generalization. Since these operations aren't masked, we can ignore the mask policy bit when deciding on compatibility. The previous code was overly strict in checking that both policy bits matched.

Note: There's a slight difference from the reviewed version.  The reviewed version was based on a local revision which included the isCompatible change to only check AVL if VL is used.  I apparently never landed that change, and while functional, the functional change isn't visible without this one.  I chose to role the extra change into this patch.

Differential Revision: https://reviews.llvm.org/D140147