Compiler warning for "condition is always true"

[pcwalton]

while (i >= 0u) cost a morning. We should add a warning for this.

[brson]

We have some functions for evaluating const expressions, I believe in ast_util that could be used for this.

[catamorphism]

Also see #1417

[veddan]

Some simple cases can be handled by #3833.

[graydon]

Type limits catches this case. If others show up we're not catching, we can look at them as they arise. Closing.