MINOR: compiler: add a new "ASSUME" macro to help the compiler

MINOR: compiler: add a new "ASSUME" macro to help the compiler

This macro takes an expression, tests it and calls an unreachable
statement if false. This allows the compiler to know that such a
combination does not happen, and totally eliminate tests that would
be related to this condition. When the statement is not available
in the compiler, we just perform a break from a do {} while loop
so that the expression remains evaluated if needed (e.g. function
call).

(cherry picked from commit efc897484b8b875f908c144d82313923be8c3392)
Signed-off-by: Willy Tarreau <w@1wt.eu>