[LV] Ignore more debug info.

We can avoid doing some unnecessary work by skipping debug instructions
in a few loops. It also helps to ensure debug instructions do not
prevent vectorization, although I do not have any concrete test cases
for that.

Reviewers: rengolin, hsaito, dcaballe, aprantl, vsk

Reviewed By: rengolin, dcaballe

Differential Revision: https://reviews.llvm.org/D53091

llvm-svn: 344232