[SimplifyCFG] 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: aprantl, vsk, hans, danielcdh

Reviewed By: hans

Differential Revision: https://reviews.llvm.org/D46252

llvm-svn: 331221