[InstCombine] Add a DEBUG_COUNTER to InstCombine to limit how many...

[InstCombine] Add a DEBUG_COUNTER to InstCombine to limit how many instructions are visited for debug

Sometimes it would be nice to stop InstCombine mid way through its combining to see the current IR. By using a debug counter we can place an upper limit on how many instructions to process.

This will also allow skipping the first X combines, but that has the potential to change later combines since earlier canonicalizations might have been skipped.

Differential Revision: https://reviews.llvm.org/D36553

llvm-svn: 310638