nir: Get rid of function overloads

When Connor originally drafted NIR, he copied the same function+overload
system that GLSL IR had with a few names changed.  However, this
double-indirection is not really needed and has only served to confuse
people.  Instead, let's just have functions which may not have unique names
and may or may not have an implementation.  If someone wants to do overload
resolving, they can hav a hash table based function+overload system in the
overload resolving pass.  There's no good reason to keep it in core NIR.

Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Acked-by: Kenneth Graunke <kenneth@whitecape.org>
ir3 bits are

Reviewed-by: Rob Clark <robclark@gmail.com>