[LV] Use BB::instructionsWithoutDebug to skip DbgInfo (NFC).

This patch updates some code responsible the skip debug info to use
BasicBlock::instructionsWithoutDebug. I think this makes things
slightly simpler and more direct.

Reviewers: mkuper, rengolin, dcaballe, aprantl, vsk

Reviewed By: rengolin

Differential Revision: https://reviews.llvm.org/D46254

llvm-svn: 331174