Preserve 'nonnull' when changing type of the load.

When changing the type of a load in Chandler's recent InstCombine changes, we can preserve the new 'nonnull' metadata.

I considered adding an assert since 'nonnull' is only valid on pointer types, but casting a pointer to a non-pointer would involve more than a bitcast anyways.  If someone extends this transform to handle more than bitcasts, the verifier will report the malformed IR, so a separate assertion isn't needed.  Also, the fpmath flags would have the same problem.