gallivm: add early cse pass

gallivm: add early cse pass

This pass is quite cheap, and can simplify the IR quite a bit for our
generated IR.
In particular on a variety of shaders I've found the time saved by
other passes due to the simplified IR more than makes up for the cost
of this pass, and on top of that the end result is actually better.
The only downside I've found is this enables the LICM pass to move some
things out of the main shader loop (in the case I've seen, instanced
vertex fetch (which is constant within the jit shader) plus the derived
instructions in the shader) which it couldn't do before for some reason.
This would actually be desirable but can increase compile time
considerably (licm seems to have considerable cost when it actually can
move things out of loops, due to alias analysis). But blaming early cse
for this seems inappropriate. (Note that the first two sroa / earlycse
passes are similar to what a standard llvm opt -O1/-O2 pipeline would
do, albeit this has some more passes even before but I don't think
they'd do much for us.)
It also in particular helps some crazy shader used for driver
verification (don't ask...) a lot (about factor of 6 faster in compile
time) (due to simplfiying the ir before LICM is run).
While here, also move licm behind simplifycfg. For some shaders there
seems to be very significant compile time gains (we've seen a factor
of 10000 albeit that was a really crazy shader you'd certainly never
see in a real app), beause LICM is quite expensive and there's cases
where running simplifycfg (along with sroa and early-cse) before licm
reduces IR complexity significantly. (I'm not entirely sure if it would
make sense to also run it afterwards.)

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>