[AArch64] Add DAG combine for extract extend pattern

This patch adds a DAG combine for (any_extend (extract_vector_elt v, i)) ->
(extract_vector_elt v, i). The combine enables us to better match some SMOV
patterns.

Differential Revision: http://reviews.llvm.org/D15515

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255895 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed