agx: Ensure that the optimizer sees legitimate SSA

Expecting it to keep around unused definitions around is wishful. Add an
"anchoring" unit_test instruction to consume the results so they don't
have to be precoloured registers.

Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/18687>

src/asahi/compiler/agx_opcodes.py

@@ -272,3 +272,5 @@ op("p_split", _, srcs = 1, dests = 4)
 # Phis are special-cased in the IR as they (uniquely) can take an unbounded
 # number of source.
 op("phi", _, srcs = 0)
+
+op("unit_test", _, dests = 0, srcs = 1, can_eliminate = False)