[RISCV] Use structured bindings in common RVV lowering code

This patch uses structured bindings to simplify a couple of specific
cases when lowering RVV operations where we commonly declare two
SDValues and immediately 'tie' them to the mask and vector length.

There's also a couple places where we split vectors that structured
bindings make sense to use.

This patch tries to keep these sorts of changes minimal and to cases
where the returned types are commonly understood, rather than applying
this wholesale to the RISCV backend.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D134442