[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

llvm-svn: 363658