Use LLVM SCEV for loops

[mcopik]

We didn't finish the implementation that would use SCEV for all loops. We should use LLVM's scalar evolution to generate expressions describing the iteration count of loop and make it taint sink instead of checking loop exit conditions in every iteration.

It has to be enabled with a switch to allow for future benchmarking.

• Run SCEV analysis
• Print statistics on constant loops
• Add SCEV test
• Skip instrumentation of constant loop
• Generate iteration count expression for non-constant loops