Register renaming pass in gcc

[Stan Cox]

One example of where it helps is to rename predicate registers to
improve predication.  

The algorithm takes output dependencies into account.  On the x86 it
is not doing much renaming because it is renaming basic blocks, not
extended basic blocks, and it doesn't have many registers to work
with.  Improving the dependency analysis for extended basic blocks and
continuing to rename registers in a round robin fashion should help
things on the x86.