missed optimization with return statement in function

Consider this trivial shader:

```
vector foo(string a)
{
if( a == "" )
return 0;
return 1;
}

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

running `testshade --debug simple.oso` yields, among a wall of text:
```
Optimized 8 ops to 6 (-25.0%)
Globals written: (1) P
```
A simple change to the function:
```
vector foo(string a)
{
if( a == "" )
return 0;
else
return 1;
}
```

and the output is now:
```
Optimized 9 ops to 1 (-88.9%)
Globals written: (0)
```
I would expect the original function to allow the same optimization.

### 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

Start by reproducing the two shader variants with testshade --debug simple.oso and compare their optimization output. Trace the optimization handling for return statements, then add a regression test showing that the original form receives the same optimization as the explicit else form.

Written by the indexing model from the issue text.