[SelectionDAG] Fix getVectorSubVecPointer for scalable subvectors.

When inserting a scalable subvector into a scalable vector through
the stack, the index to store to needs to be scaled by vscale.
Before this patch, that didn't yet happen, so it would generate the
wrong offset, thus storing a subvector to the incorrect address
and overwriting the wrong lanes.

For some insert:
  nxv8f16 insert_subvector(nxv8f16 %vec, nxv2f16 %subvec, i64 2)

The offset was not scaled by vscale:
  orr     x8, x8, #0x4
  st1h    { z0.h }, p0, [sp]
  st1h    { z1.d }, p1, [x8]
  ld1h    { z0.h }, p0/z, [sp]

And is changed to:
  mov x8, sp
  st1h { z0.h }, p0, [sp]
  st1h { z1.d }, p1, [x8, #1, mul vl]
  ld1h { z0.h }, p0/z, [sp]

Differential Revision: https://reviews.llvm.org/D111633