[AArch64][DAGCombine] Add performBuildVectorCombine 'extract_elt ~> anyext'

A build vector of two extracted elements is equivalent to an extract
subvector where the inner vector is any-extended to the
extract_vector_elt VT, because extract_vector_elt has the effect of an
any-extend.

  (build_vector (extract_elt_i16_to_i32 vec Idx+0) (extract_elt_i16_to_i32 vec Idx+1))
  => (extract_subvector (anyext_i16_to_i32 vec) Idx)

Depends on D130697

Differential Revision: https://reviews.llvm.org/D130698