ir_constant_expression: Convert from a visitor to a virtual function.

ir_constant_expression: Convert from a visitor to a virtual function.

The constant_expression_wrapper was already the only external API, and
much of the internal code used it anyway.  Also, it wouldn't ever visit
non-rvalue ir_instructions, so using a visitor seemed a bit unnecessary.

This uses "ir_foo *ir = this;" lines to avoid code churn.  These should
be removed.