Fold FMix feeding Extract, and use the simplification pass.

We add a new rule to the folding rules to fold an FMix feeding an
extract when the alpha value for the element being extracted is either
0 or 1.  In those case, we can simple extract from one of the operands
to the FMix.

With that change the simplification pass completely subsumes the
insert-extract elimination pass.  So we remove the insert-extract
elimination passes and replce them with calls to the simplification
pass.

In a follow up PR, we should delete the insert-extract elimination pass.

Contributes to https://github.com/KhronosGroup/SPIRV-Tools/issues/1570.