[Relay] Legalize pass (#3672)

[Relay] Legalize pass (#3672)

* [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.