Move initialization of the const mgr to the constructor.

Move initialization of the const mgr to the constructor.

The current code expects the users of the constant manager to initialize
it with all of the constants in the module.  The problem is that you do
not want to redo the work multiple times.  So I decided to move that
code to the constructor of the constant manager.  This way it will
always be initialized on first use.

I also removed an assert that expects all constant instructions to be
successfully mapped.  This is because not all OpConstant* instruction
can map to a constant, and neither do the OpSpecConstant* instructions.

The real problem is that an OpConstantComposite can contain a member
that is OpUndef.  I tried to treat OpUndef like OpConstantNull, but this
failed because an OpSpecConstantComposite with an OpUndef cannot be
changed to an OpConstantComposite.  Since I feel this case will not be
common, I decided to not complicate the code.

Fixes #1193.