[NFC][SVE] Change useSVEForFixedLengthVectorVT to allow unconditional SVE usage for...

[NFC][SVE] Change useSVEForFixedLengthVectorVT to allow unconditional SVE usage for NEON sized vectors.

Previously useSVEForFixedLengthVectorVT only allowed SVE usage when
the target SVE register length was known to be at least 256bit.
This was true even for NEON sized vectors, which was an artificial
restriction imposed during early SVE bring up.  This now changes so
that callers can opt to use SVE for NEON sized vectors regardless
of the SVE register length.

The patch is NFC because for all places where OverrideNEON is used
we now explicitly also check that SVE code generation for larger
than NEON vectors is enabled.  The intent is that over time these
extra checks will either be removed or the lowering disabled if the
SVE usage proves not beneficial.

Differential Revision: https://reviews.llvm.org/D118957