[Polly] Reject scalable vector types (#177871)

Polly currently does not consider types without fixed length, which can
be encountered if an input source uses e.g. ARM SVE builtins. Such
programs have already been optimized manually. Non-fixed type lengths
also add to the difficulty of dependency analysis. Skip such types
entirely for now.

Fixes: #177859
GitOrigin-RevId: e5d235831dfb07b6e767812af7f9d3daa5896f78