[RISCV] Fix logic for determining RVV stack padding

We must add padding when using SP or BP to access stack objects.
Checking whether we're missing FP is not sufficient as stack realignment
uses SP too. The test in D125962 explains the specific issue in more
detail.

Split from D125787.

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D125964