[codegen] Add generic functions to skip debug values.

Summary:
This commits moves skipDebugInstructionsForward and
skipDebugInstructionsBackward from lib/CodeGen/IfConversion.cpp
to include/llvm/CodeGen/MachineBasicBlock.h and updates
some codgen files to use them.

This refactoring was suggested in https://reviews.llvm.org/D27688
and I thought it's best to do the refactoring in a separate
review, but I could also put both changes in a single review
if that's preferred.

Also, the names for the functions aren't the snappiest and
I would be happy to rename them if anybody has suggestions. 

Reviewers: eli.friedman, iteratee, aprantl, MatzeB

Subscribers: MatzeB, llvm-commits

Differential Revision: https://reviews.llvm.org/D27782

llvm-svn: 289933