[RISCV][InsertVSETVLI] Rework code structure to make reasoning about undefined lanes explicit [NFC]

We already have several places in this code which reason about whether the inactive lanes are defined, and are about to add one more in D151653. Let's go ahead and common the code so that we don't have the same concept repeating in multiply places.

Differential Revision: https://reviews.llvm.org/D152844