vc4: Disable vc4_opt_vpm in the presence of control flow.

It's a really valuable pass currently, but it will be a mess to rewrite
for control flow.  For now, just disable it if we have multiple blocks
present.

src/gallium/drivers/vc4/vc4_opt_vpm.c

@@ -38,6 +38,11 @@ qir_opt_vpm(struct vc4_compile *c)
         if (c->stage == QSTAGE_FRAG)
                 return false;
 
+        /* For now, only do this pass when we don't have control flow. */
+        struct qblock *block = qir_entry_block(c);
+        if (block != qir_exit_block(c))
+                return false;
+
         bool progress = false;
         struct qinst *vpm_writes[64] = { 0 };
         uint32_t use_count[c->num_temps];