[IR] Introduce helpers to skip debug instructions (NFC)

This patch introduces two helpers to make it easier to ignore debug
intrinsics:

- Instruction::getNextNonDebugInstruction()

This is just like Instruction::getNextNode(), except that it skips debug
info.

- skipDebugInfo(BasicBlock::iterator)

A free function which advances a BasicBlock iterator past any debug
info. This is a no-op when the iterator already points to a non-debug
instruction.

Part of: llvm.org/PR37728
Related to: https://reviews.llvm.org/D47874

Differential Revision: https://reviews.llvm.org/D48305

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335083 91177308-0d34-0410-b5e6-96231b3b80d8