nir: Add some ssa-only fast-paths for nir_src rewrite

nir: Add some ssa-only fast-paths for nir_src rewrite

Basically every pass in NIR uses nir_ssa_def_rewrite_uses which calls
nir_instr_rewrite_src which is fairly complex because it handles all
sorts of non-SSA cases.  Since we already know a priori that every
source written by nir_ssa_def_rewrite_uses is SSA, we can check new_src
once at the top of the function and cut out all that complexity.

While we're at it, we expose a new SSA-only nir_ssa_def_rewrite_uses_ssa
helper which takes an SSA def which avoids the one SSA check.  It's also
more convenient 90% of the time.

Compile time as tested by Rhys Perry <pendingchaos02@gmail.com>

    Difference at 95.0% confidence
        -797.166 +/- 418.649
        -0.566174% +/- 0.296441%
        (Student's t, pooled s = 325.459)

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Connor Abbott <cwabbott0@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8790>