Remove ValueData::Alias

[sbillig]

The new parser's use of aliases revealed some alias-related bugs in some of the optimizations. The cleanest fix for this is to just remove ValueData::Alias, and instead replace the uses of the alias Value with the original Value (which is easy, because the dfg already tracks which instructions use each Value.)

I also changed the parser so that it no longer uses aliases. The prior method of handling values in the parser lead to the wrong type being assigned to some insn result values (in cases where the first argument to some insn was a value whose definition had not yet been seen by the parser). The new method is to first do a pass over all statements in a function to "declare" all values with their proper types (by calling dfg.make_value with dummy ValueData).

[Y-Nak]

LGTM!