[PSI2IR] Propagate smart cast information for variable loads.

This gives us more precise type information and can enable backend
optimizations. This was motivated by when expressions not compiled
to table switches in the JVM_IR backend.

Fixed KT-36845.

compiler/testData/ir/irText/expressions/floatingPointComparisons/nullableAnyAsIntToDouble.kt.txt

@@ -1,12 +1,6 @@
 fun test(x: Any?, y: Double): Boolean {
   return when {
-    x is Int -> less(arg0 = { // BLOCK
-      val tmp0_safe_receiver: Any? = x
-      when {
-        EQEQ(arg0 = tmp0_safe_receiver, arg1 = null) -> null
-        else -> tmp0_safe_receiver /*as Int */.toDouble()
-      }
-    }, arg1 = y)
+    x is Int -> less(arg0 = x /*as Int */.toDouble(), arg1 = y)
     else -> false
   }
 }