[SCEVExp] Advance found insertion point until we find a non-dbg instruction.

This avoids creating unnecessary casts if the IP used to be a dbg info
intrinsic. Fixes PR37727.

Reviewers: vsk, aprantl, sanjoy, efriedma

Reviewed By: vsk, efriedma

Differential Revision: https://reviews.llvm.org/D47874

llvm-svn: 335513