Remove redundant stores.

The code patterns generated by DXC around function calls can cause many
store to be storing the same value that was just loaded from the same
location:

```
%10 = OpLoad %type %var
OpStore %var %10
```

We want to clean these up very early on because they can cause other
transformations to do a lot of work.  For the cases I see, they can be
removed during local-single-block-elim.

For one set of shaders the compile time goes from 248s to 182s.  A 26%
improvement.

Part of https://github.com/KhronosGroup/SPIRV-Tools/issues/1494.