[ConstraintElim] Add reproducer remarks.

This patch adds an optimization remark for each performed optimization
containing a module that can be used to reproduce the transformation.

The reproducer function contains a series of @llvm.assume calls, one for
each condition currently in scope. For each condition, the operand
instruction are cloned until we reach operands that have an entry in the
constraint system. Those will then be added as function arguments.

The reproducer functions are minimal, that is, they only contain the
conditions required for a given simplification. The resulting IR is very
compact and can be used to verify each transformation individually.

It also provides a python script to extract the IR from the remarks and
create LLVM IR files from it.

Reviewed By: paquette

Differential Revision: https://reviews.llvm.org/D143323