RFC: Using mode and code macros in *.md files

[Joern Rennecke]

> Hopefully, with optimizations moving up into trees and SSA the RTL
> optimizations won't come into play as much.

The tree optimizers can't give you optimizations that have to do with
instruction selection, since they are mostly agnostic of your target
instruction set.  The tree optimizers can do a good job to do optimizations
on the abstract computations, but the rtl optimizers like combine are
important to map these computations efficiently on your target
instruction set.