[AArch64] Fix condition for "high-vector" DUP optimizations.

AArch64 NEON has a bunch of instructions with a "2" suffix that extract
the top half of the source vectors, instead of the bottom half.  We have
some DAGCombines to try to take advantage of that.  However, they
assumed that any EXTRACT_VECTOR was extracting the high half of the
vector in question.

This issue has apparently existed since the AArch64 backend was merged.

Fixes https://bugs.llvm.org/show_bug.cgi?id=40632 .

Differential Revision: https://reviews.llvm.org/D57862

llvm-svn: 353486