[BranchFolding] skip debug instr to avoid code change

Use the existing helper function in BranchFolding, "countsAsInstruction",
to skip over non-instructions. Otherwise debug instructions can be
identified as the last real instruction in a block, leading to different
codegen decisions when debug is enabled as demonstrated by the test case.

Patch by: yechunliang (Chris Ye)!

Differential Revision: https://reviews.llvm.org/D66467