* [Relay] Rewrite pass.
This pass transforms an expression to other expression.
This pass has many usecases
* Replace a expr to another expr, if the other expr has faster performance.
* For ASICs, we might want to modify the inputs to adapt to the HW support.
* Alter op layout can work in conjunction with this pass.
The supporting usecase is the Intel i8 x i8 conv. Intel HW supports u8 x i8 conv
in HW. Using this pass, we can replace an i8 x i8 conv to a sequence of
operators where one of the operators is now u8 x i8 conv. This will also help
automatic quantizaion performance.
* Better API name.
* Removing the conv2d legalization for x86. Will send a separate PR.
* Test name changes.
* Registering one funtion to register FTVMLegalize.
* Better comments.
