[SVE] Lower fixed length VECREDUCE_[SMAX|SMIN] to Scalable

This patch is pretty similar to the VECREDUCE_ADD patch, with some minor tweaks.

Results from the AArch64ISD::[SMAX|SMIN]V_PRED return element sized results. This requires an ANY_EXTEND for results < 32-bits, since Legalization promotes those results.

There is no NEON i64 vector support for SMAXV|SMINV, so use SVE for those.

Differential Revision: https://reviews.llvm.org/D88259