[Spesh] unbreak rakudo by not optimizing isconcrete in istrue

This can reduce the isconcrete to a constant value, which is cool, but
which seems to break rakudo CORE.setting building under conditions:

MVM_SPESH_OSR_DISABLE=1
MVM_SPESH_INLINE_DISABLE=1
MVM_JIT_DISABLE=1
MVM_SPESH_BLOCKING=1
MVM_SPESH_LIMIT=1437

Will investigate later *why* this breaks things, but I'm guessing there
are guards that are not properly installed because of the use of a
temporary register.

src/spesh/optimize.c

@@ -959,7 +959,7 @@ static void optimize_istrue_isfalse(MVMThreadContext *tc, MVMSpeshGraph *g, MVMS
         case MVM_BOOL_MODE_NOT_TYPE_OBJECT:
             ins->info = MVM_op_get_op(MVM_OP_isconcrete);
             /* And now defer another bit of optimization */
-            optimize_isconcrete(tc, g, ins);
+//            optimize_isconcrete(tc, g, ins);
             break;
             /* We need to change the register type for our result for this,
              * means we need to insert a temporarary and a coerce: