[Bug tree-optimization/109637] unnecessary range check in complete switch on bitfield

["pinskia at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109637

--- Comment #7 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
(In reply to Andrew Macleod from comment #2)
> 
> Why are we adding -1 to [0,3] ?  Thats the root of this issue I think? 
> seems strange

if we ignore the range for a second.

When switch conversion happens we have:
  switch (_1) <default: <L5> [INV], case 1: <L1> [INV], case 2: <L2> [INV],
case 3: <L3> [INV]>

So when _1 is either 0 or > 3, default case will happen.

The real problem is switch conversion promotes to a 8 bit type when it does the
comparisons and does not take into the ranges.

Related testcase (ignoring that there is a bogus warning, see PR 95513 for that
and not fixed even with my VRP patch):
```
int f (unsigned t) {
    switch(t & 0x3) {
        case 0: return 0;
        case 1: return 1;
        case 2: return 2;
        case 3: return 3;
    }
}
```
my VRP patch also fixes the above. Now switch conversion should use the range
info too. I will look into adding that .... But at least we get decent code for
the orginal and related cases now.