Simplify float or integer constant to 0 or 1

When manually simplifying a spirv-reduced shader I found that I could change:

```
%11 = OpConstant %7 0.600000024
```

to

```
%11 = OpConstant %7 0
```

without changing the interestingness of the shader, suggesting that spirv-reduce is not doing this simplification.

Start at the spirv-reduce constant-simplification logic and use the reduced shader example in the issue as the reproduction case. Trace how float and integer constants are considered for simplification, then verify that eligible constants can become 0 or 1 without changing the shader's interestingness.

Written by the indexing model from the issue text.