Fold OpDot

Adding three rules to fold OpDot (implemented as two).

- When an OpDot has two constants, then fold to the resulting const.

- When one of the inputs is the 0 vector, then fold to zero.

- When one of the inputs is a single 1 with 0s, then rewrite to an
OpCompositeExtract of the appropriate element.  This will help find
even more folding opportunities.

Contributes to #709.