[RISCV] Ensure the entire stack is aligned to the RVV stack alignment

This patch fixes another bug in the RVV frame lowering. While some frame
objects with non-default stack IDs (such scalable-vector alloca
instructions) are considered in the target-independent max alignment
calculations, others (for example, during calling-convention lowering)
are not. This means we'd occasionally align the base of the stack to
only 16 bytes, with no way to ensure that the RVV section contained
within that is aligned to anything higher.

Reviewed By: StephenFan

Differential Revision: https://reviews.llvm.org/D125973