[InstCombine] Allow to limit the max number of iterations

Summary:
This patch teaches InstCombine to accept a new parameter: maximum number of iterations over functions.

InstCombine tries to simplify instructions by iterating over the whole function until the function stops changing. As a consequence, the last iteration before reaching a fixpoint visits all instructions in the worklist and never performs any rewrites.

Bounding the number of iterations can have 2 benefits:
* In case the users of the pass can make a good guess about the number of required iterations, we can save the time normally spent on the last iteration that doesn't change anything.
* When the wants to use InstCombine as a cleanup pass, it may be enough to run just a few iterations and stop even before reaching a fixpoint. This can be also useful for implementing a lightweight pass pipeline (think `-O1`).

This patch does not change the behavior of opt or Clang -- limiting the number of iterations is entirely opt-in.

Reviewers: fhahn, davide, spatel, foad, nlopes, grosser, lebedev.ri, nikic, xbolva00

Reviewed By: spatel

Subscribers: craig.topper, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71145

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