[RISCV] Emit DWARF location expression for RVV stack objects.

VLENB is the length of a vector register in bytes. We use
<vscale x 64 bits> to represent one vector register. The dwarf offset is
VLENB * scalable_offset / 8.

For the mask vector, it occupies one vector register.

Differential Revision: https://reviews.llvm.org/D107432