[AArch64] fold 'isPositive' vector integer operations (PR26819)

This is one of the cases shown in:
https://llvm.org/bugs/show_bug.cgi?id=26819

Shift and negate is what InstCombine prefers to produce (and I tried to make it do more of that
in http://reviews.llvm.org/rL262424 ), so we should recognize that pattern as something that might
come from autovectorization even if it's unlikely to be produced from C NEON intrinsics.

The patch is based on the x86 equivalent:
http://reviews.llvm.org/rL262036

Differential Revision: http://reviews.llvm.org/D17834

llvm-svn: 262623