missed optimization with "complex" condition

This is similar to #1146 but likely has a different root cause. Consider this shader:
```
vector foo(float x, string a)
{
vector result = 0;
if( x >= -1 && a != "" )
{
result = 1;
}
return result;
}

surface simple(string s = "")
{
P += foo(u, s);
}

```
Once again, `testshade --debug simple.oso` yields:
```
Globals written: (1) P
```
Making the condition simpler:
```
vector foo(float x, string a)
{
vector result = 0;
if( a != "" )
{
result = 1;
}
return result;
}
```
is enough to get proper optimization:
```
Globals written: (0)
```

As that second condition can clearly be proven false, the first one should be easy to prove false also. So I would expect it to be optimized the same way.

### Versions

* OSL branch/version: 1.10.9 (testshade output above) / 3d0a284e997367b5b63bf8ea16911391fa1fea85 (where I originally found the issue in a larger network)
* OS: linux
* LLVM version: 9.0.1 / 6.0.0

Reproduce the shader from the issue with `testshade --debug simple.oso`, comparing the complex condition with the simpler `a != ""` condition. Trace the optimization path responsible for proving the condition false, and confirm the fix makes the first case report `Globals written: (0)` as well.

Written by the indexing model from the issue text.