[AArch64][SVE] Fix extract_subvector patterns for unpacked fp types.

The patterns added in D110163 were incorrect, since it used the wrong
element widths for its shuffles.

Example for nxv2f16 extract_subvector(nxv8f16 %in, 6):
  <a|b|c|d|e|f|g|h>
               ^^^
           extract g and h.

  => UUNPKHI .h -> .s results in:
  <e  |f  |g  |h  >

  => UUNPKHI .s -> .d results in:
  <g      |h      >

Reviewed By: david-arm

Differential Revision: https://reviews.llvm.org/D110523