Lessen the number of spesh threshold levels

When something tiny called something a bit bigger, we could end up
with the call there not being able to specialize into a fast resolve
due to the callee potentially not being hot enough yet. Once things
get over a certain size that'll be noise, but there's plenty of cases
(tiny forwarding subs/methods delegating to slightly bigger impls)
that could benefit. So, collapse the lower levels to a single one.

src/spesh/threshold.c

@@ -6,14 +6,10 @@ MVMuint32 MVM_spesh_threshold(MVMThreadContext *tc, MVMStaticFrame *sf) {
     MVMuint32 bs = sf->body.bytecode_size;
     if (tc->instance->spesh_nodelay)
         return 1;
-    if (bs <= 256)
-        return 100;
-    else if (bs <= 512)
+    if (bs <= 2048)
         return 150;
-    else if (bs <= 2048)
-        return 200;
     else if (bs <= 8192)
-        return 250;
+        return 200;
     else
         return 300;
 }