Fix "fptoui xxx to i32" to use a helper.
Reformat.
Add A/B tests for the cast operators.

Several things are still needed to complete this test:

1. The cast*ToUi1() functions need to be rewritten as .ll because
clang generates icmp/fcmp instead of trunc/fptoui.

2. Several casts between integer and floating-point are commented out
because the required helpers are not yet there.  Also, something is
broken about the way Subzero generates the external helper
declarations -- right now it seems to be generating bogus definitions
instead of just declarations.

3. Come up with more interesting floating-point test values.

4. Try to rewrite this whole test framework using templates,to avoid
the macros and massive code duplication.