Fix PR 33738

[Diego Novillo]

This bug is a failure to warn about an always-false predicate when
-Wtype-limits is used.  Given:

enum Alpha {
 ZERO = 0, ONE, TWO, THREE
};

Alpha a2;

int m1 = -1;
int GetM1() {
 return m1;
}

int main() {
 a2 = static_cast<Alpha>(GetM1());
 if (a2 == -1) {        // { dg-warning "always false due" }
    link_error ();
 }
 if (-1 == a2) {        // { dg-warning "always false due" }
    link_error ();
 }
 return 0;
}

The front end misses the warning because it explicitly upcasts both
operands when doing the checks.  However, during VRP we discover that
a2 can never actually take the value -1, so the predicate is
discarded.  With this patch, if -Wtype-limits is used, we also emit a
warning about it.

Bootstrapped and tested on x86_64, ppc64 and i686.


Diego.
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