[LoopVectorize][AArch64] Add TTI hook preferPredicatedReductionSelect

By default if SVE is enabled we want the select instruction used for
reductions to be inside the loop, rather than outside. This makes it
possible for the backend to fold the select into the operation to
produce a single predicated add, fadd, etc.

Differential Revision: https://reviews.llvm.org/D129763