[AArch64] Add EXT patterns for 64-bit EXT of a subvector of a 128-bit vector

If we have a 64-bit EXT where one of the operands is a subvector of a 128-bit
vector then in some cases we can eliminate an extract_subvector by converting
to a 128-bit EXT of the 128-bit vector.

Differential Revision: https://reviews.llvm.org/D53582

llvm-svn: 345275