This reverts commit 2904c619c1.
In order to support swizzle operations, I need to be able to alias defs
to larger types (eg, float to vec4), but alias_def rightly won't allow
this. However, as the plan is to do this in the final steps before
emitting the instruction, I plan on creating an alias to a float then
adjusting the type in the alias, but to do so without extra shenanigans,
I need alias_def to allow aliases to the same type. As a fringe benefit,
it makes the code agree with the comment in def.h :P
3c20dd515e