Enable the IR checker post optimizer with RT longs

[gzm0]

The first two commits are not strictly necessary. However, I felt they significantly help understanding of what the optimizer is doing.

[gzm0]

It looks like the change in subtyping checks (3rd commit) causes long literals to become allocated as RT long:

 $c_jl_Math$.prototype.multiplyExact__J__J__J = (function(a, b) {
   var ahi = b.RTLong__f_hi;
   if (((ahi === 0) ? (b.RTLong__f_lo !== 0) : (ahi > 0))) {
-    var this$1 = $m_RTLong$();
-    var lo = this$1.divideImpl__I__I__I__I__I((-1), 2147483647, b.RTLong__f_lo, b.RTLong__f_hi);
-    var hi = this$1.RTLong$__f_org$scalajs$linker$runtime$RuntimeLong$$hiReturn;
+    var this$1 = new $c_RTLong((-1), 2147483647);
+    var this$2 = $m_RTLong$();
+    var lo = this$2.divideImpl__I__I__I__I__I(this$1.RTLong__f_lo, this$1.RTLong__f_hi, b.RTLong__f_lo, b.RTLong__f_hi);
+    var hi = this$2.RTLong$__f_org$scalajs$linker$runtime$RuntimeLong$$hiReturn;
     var ahi$1 = a.RTLong__f_hi;
     if (((ahi$1 === hi) ? (((-2147483648) ^ a.RTLong__f_lo) > ((-2147483648) ^ lo)) : (ahi$1 > hi))) {
       var overflow = true;

I'm trying to identify where the "offending" subtyping check is in the optimizer.

[gzm0]

I have not gotten to a conclusion here yet. But it seems the issue is due to casts inserted on the receiver when inlining method calls: We need these when, say, calling + on a literal long for the IR to be correctly typed. OTOH, inserting them (which is what the 3rd commit causes) prevents farther inlining and folding of runtime long methods.

[gzm0]

I have managed to fix the execution tests: the claim that we never do an isInstance check on RT long was wrong: RT long itself does one in its equals method. As a result, when inlining x.equals(y) with longs, the compiler generated wrong folded constants.

I have not figured out yet what causes long literals to be stack allocated rather than inlined like before.