Add phase FlowSensitiveReductionPhase. It is possible to remove GuardingPiNodes, CheckCastNodes, and FixedGuards during HighTier under certain conditions (control-flow sensitive conditions). The phase added in this commit (FlowSensitiveReductionPhase) does that, and in addition replaces usages with "downcasting" PiNodes when possible thus resulting in more precise object stamps (e.g., non-null). Finally, usages of floating, side-effects free, expressions are also simplified (as per control-flow sensitive conditions). The newly added phase runs only during HighTier and can be deactivated using Graal option FlowSensitiveReduction (it is active by default).