pan/bi: Allow CSE of preloaded registers

Needed to CSE `LEA_VARY` in varying shaders on Valhall.

No shader-db changes on Bifrost.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/15204>

src/panfrost/bifrost/bi_opt_cse.c

@@ -139,9 +139,12 @@ instr_can_cse(const bi_instr *I)
                         return false;
         }
 
-        /* Similar refuse to CSE non-SSA sources */
+        /* Similar refuse to CSE non-SSA sources. We allow machine registers,
+         * since CSE runs before register allocation which means any registers
+         * encountered are preloaded and hence assumed constant.
+         */
         bi_foreach_src(I, s) {
-                if (I->src[s].reg || I->src[s].type == BI_INDEX_REGISTER)
+                if (I->src[s].reg)
                         return false;
         }