[AArch64] Improve ISel using across lane addition reduction.

In vectorized add reduction code, the final "reduce" step is sub-optimal.
This change wll combine :

ext  v1.16b, v0.16b, v0.16b, #8
add  v0.4s, v1.4s, v0.4s
dup  v1.4s, v0.s[1]
add  v0.4s, v1.4s, v0.4s

into

addv s0, v0.4s

PR21371
http://reviews.llvm.org/D12325
Patch by Jun Bum Lim <junbuml@codeaurora.org>!

llvm-svn: 246790