This transformation, given the header of a selection construct with
branching instruction OpBranchConditional, flattens it.
Side-effecting operations such as OpLoad, OpStore and OpFunctionCall
are enclosed within smaller conditionals.
It is applicable if the construct does not contain inner selection
constructs or loops, or atomic or barrier instructions.
The corresponding fuzzer pass looks for selection headers and
tries to flatten them.
Needed for the issue #3544, but it does not fix it completely.
