Added AST and Function classes.
Now, we parse the whole file and build up the AST for all of the
functions in the Module before we emit IR for the functions (vs. before,
when we generated IR along the way as we parsed the source file.)
Go back to running both sides of 'if' statements with masking and without
branching if we can determine that the code is relatively simple (as per
the simple cost model), and is safe to run even if the mask is 'all off'.
This gives a bit of a performance improvement for some of the examples
(most notably, the ray tracer), and is the code that one wants generated
in this case anyhow.
This is currently only used to decide whether it's worth doing an
"are all lanes running" check at the start of functions--for small
functions, it's not worth the overhead.
The cost is estimated relatively early in compilation (e.g. before
we know if an array access is a scatter/gather or not, before
constant folding, etc.), so there are many known shortcomings.
For the case where we have a regular (i.e. non-'cif') 'if' statement,
the generated code just simply checks to see if any program instance
is running before running the corresponding statements. This is a
lighter-weight check than IfStmt::emitMaskMixed() was performing.
Contrary to claims in 0c2048385, that checkin didn't include the changes
to not run if/else blocks if none of the program instances wanted to be
running them. This checkin fixes that and thus actually fixes issue #74.
