Prefix Graph(Tree) Config API

It's possible to add a prefix graph API to fine granularly control circuit PPA and optimize for post synthesizing from Chisel:
This can benefit Add/Sub LeftOr/RightOr and other APIs

**Type of issue**: feature request

**Impact**: API addition (no impact on existing code)

**Development Phase**: proposal

**Other information**

**If the current behavior is a bug, please provide the steps to reproduce the problem:**

**What is the current behavior?**
expose + - * / as primitive, throw optimization effort to synthesis tools.

**What is the expected behavior?**
An prefix graph API to let user fine tune the adder(or other circuit) that is used by Chisel.

**Please tell us about your environment:**
Chisel3.6-SNAPSHOT

**What is the use case for changing the behavior?**
fine tune to combinational circuit

Start by reviewing the existing Add/Sub LeftOr/RightOr APIs and how Chisel currently exposes primitive +, -, *, and / operations. Define the proposed prefix graph API and its user-facing configuration scope, then verify that it enables circuit fine-tuning for post-synthesis optimization without affecting existing code.

Written by the indexing model from the issue text.