[aarch64] move custom isel of extract_vector_elt to td file - NFC

In preparation for def-pat selection of dot product instructions,
this patch moves the custom instruction selection of extract_vector_elt
to the td file. Without this change it is impossible to catch a pattern that
starts with an extract_vector_elt: the custom cpp code is executed first
ahead of the patterns in the td files that are only executed at the end of
the switch statement in SelectCode(Node).

With this patch applied, it becomes possible to select a different pattern
that starts with extract_vector_elt by selecting a higher complexity than
this pattern.

The patch has been tested on aarch64-linux with make check-all.

Differential Revision: https://reviews.llvm.org/D67497

llvm-svn: 371887