[LoopVectorize] Don't emit remarks about lack of scalable vectors unless...

[LoopVectorize] Don't emit remarks about lack of scalable vectors unless they're specifically requested.

Previously we emitted a "does not support scalable vectors"
remark for all targets whenever vectorisation is attempted. This
pollutes the output for architectures that don't support scalable
vectors and is likely confusing to the user.

Instead this patch introduces a debug message that reports when
scalable vectorisation is allowed by the target and only issues
the previous remark when scalable vectorisation is specifically
requested, for example:

  #pragma clang loop vectorize_width(2, scalable)

Differential Revision: https://reviews.llvm.org/D108028