This macro can be used to tell the compiler's optimization passes static
assumptions which you *know* are true about code behavior.

Use of these can be risky -- if you assume incorrectly, the compiler may
emit code that will not work in circumstances you didn't anticipate.

On the other hand, use of this macro in places where the optimizer is
missing an assumption that would have been safe to make can cause it to
emit more compact/optimal code.
Signed-off-by: Steven Noonan <steven@uplinklabs.net>