[SVE][IR] Scalable Vector IR Type with pr42210 fix
Recommit of D32530 with a few small changes:
- Stopped recursively walking through aggregates in the verifier, so that we don't impose too much overhead on large modules under LTO (see PR42210).
- Changed tests to match; the errors are slightly different since they only report the array or struct that actually contains a scalable vector, rather than all aggregates which contain one in a nested member.
- Corrected an older comment
Reviewers: thakis, rengolin, sdesmalen
Reviewed By: sdesmalen
Differential Revision: https://reviews.llvm.org/D63321