[AArch64][SVE] Only fold frame indexes referencing SVE objects into SVE loads/stores

Currently we always fold frame indexes into SVE load/store instructions,
however these instructions can only encode VL scaled offests. This means
that when we are accessing a fixed length stack object with these
instructions, the folded in frame index gets pulled back out during frame
lowering. This can cause issues when we have no spare registers and no
emergency spill slot.

Rather than causing issues like this, don't fold in frame indexes that
reference fixed length objects.

Fixes: #55041

Differential Revision: https://reviews.llvm.org/D124457