Make sure output of spesh resolve gets optimized, too

Until now we'd have resumed optimization after the last
instruction a speshresolve has put in. This misses the
opportunity to improve getattr faux-guards into direct
pointer access versions.

Fix this by remembering the previous instruction before
speshresolve is optimized and continuing with that
instruction. The next thing in the loop is to go to
ins->next, so we use prev->prev.

src/spesh/optimize.c

@@ -2319,8 +2319,19 @@ static void optimize_bb_switch(MVMThreadContext *tc, MVMSpeshGraph *g, MVMSpeshB
             optimize_call(tc, g, bb, ins, p, 1, &arg_info);
             break;
         case MVM_OP_speshresolve:
-            if (p)
+            if (p) {
+                /* Rewriting spesh plugins will insert a bunch of instructions
+                 * in front of the generated code, which would be skipped by
+                 * our loop, so we remember the previous instruction and see
+                 * to it that we run across the new instructions, too. */
+                MVMSpeshIns *prev = ins->prev;
                 optimize_plugin(tc, g, bb, ins, p);
+                if (ins->info->opcode != MVM_OP_speshresolve
+                        && ins->info->opcode != MVM_OP_sp_speshresolve) {
+                    if (prev->prev)
+                        ins = prev->prev;
+                }
+            }
             break;
         case MVM_OP_islist:
         case MVM_OP_ishash: