Devirtualize Constant::replaceUsesOfWithOnConstant.

This is part of the work to devirtualize Value.

The old pattern was to call replaceUsesOfWithOnConstant which was overridden by
subclasses.  Those could then call replaceUsesOfWithOnConstantImpl on Constant
to handle deleting the current value.

To be consistent with other parts of the code, this has been changed so that we
call the method on Constant, and that dispatches to an Impl on subclasses.

As part of this, it made sense to rename the methods to be more descriptive.  The
new name is Constant::handleOperandChange, and it requires that all subclasses of
Constant implement handleOperandChangeImpl, even if they just throw an error if
they shouldn't be called.

Reviewed by Duncan Exon Smith.

llvm-svn: 240567