[LV] Enable scalable vectorization by default for SVE cores.

The availability of SVE should be sufficient to enable scalable
auto-vectorization.

This patch adds a new TTI interface to query the target what style of
vectorization it wants when scalable vectors are available. For other
targets than AArch64, this currently defaults to 'FixedWidthOnly'.

Differential Revision: https://reviews.llvm.org/D115651