[RISCV] Add early-exit to RVV stack computation. NFCI.

This patch was split off from D126465, where an early-exit is necessary
as it checks the VLEN and that asserts that V instructions are present.

Since this makes logical sense on its own, I think it's worth landing
regardless of D126465.

Reviewed By: kito-cheng

Differential Revision: https://reviews.llvm.org/D129617