The current SSA def validation we do in nir_validate validates three
things:

 1. That each SSA def is only ever used in the function in which it is
    defined.

 2. That an nir_src exists in an SSA def's use list if and only if it
    points to that SSA def.

 3. That each nir_src is in the correct use list (uses or if_uses) based
    on whether it's an if condition or not.

The way we were doing this before was that we had a hash table which
provided a map from SSA def to a small ssa_def_validate_state data
structure which contained a pointer to the nir_function_impl and two
hash sets, one for each use list.  This meant piles of allocation and
creating of little hash sets.  It also meant one hash lookup for each
SSA def plus one per use as well as two per src (because we have to look
up the ssa_def_validate_state and then look up the use.)  It also
involved a second walk over the instructions as a post-validate step.

This commit changes us to use a single low-collision hash set of SSA
sources for all of this by being a bit more clever.  We accomplish the
objectives above as follows:

 1. The list is clear when we start validating a function.  If the
    nir_src references an SSA def which is defined in a different
    function, it simply won't be in the set.

 2. When validating the SSA defs, we walk the uses and verify that they
    have is_ssa set and that the SSA def points to the SSA def we're
    validating.  This catches the case of a nir_src being in the wrong
    list.  We then put the nir_src in the set and, when we validate the
    nir_src, we assert that it's in the set.  This takes care of any
    cases where a nir_src isn't in the use list.  After checking that
    the nir_src is in the set, we remove it from the set and, at the end
    of nir_function_impl validation, we assert that the set is empty.
    This takes care of any cases where a nir_src is in a use list but
    the instruction is no longer in the shader.

 3. When we put a nir_src in the set, we set the bottom bit of the
    pointer to 1 if it's the condition of an if.  This lets us detect
    whether or not a nir_src is in the right list.

When running shader-db with an optimized debug build of mesa on my
laptop, I get the following shader-db CPU times:

   With NIR_VALIDATE=0       3033.34 seconds
   Before this commit       20224.83 seconds
   After this commit         6255.50 seconds

Assuming shader-db is a representative sampling of GLSL shaders, this
means that making this change yields an 81% reduction in the time spent
in nir_validate.  It still isn't cheap but enabling validation now only
increases compile times by 2x instead of 6.6x.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Thomas Helland <thomashelland90@gmail.com>