Add pass to fold a load feeding an extract.

We have already disabled common uniform elimination because it created
sequences of loads an entire uniform object, then we extract just a
single element.  This caused problems in some drivers, and is just
generally slow because it loads more memory than needed.

However, there are other way to get into this situation, so I've added
a pass that looks specifically for this pattern and removes it when only
a portion of the load is used.

Fixes #1547.