[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/implicitCastToNonNull.kt.txt

@@ -15,14 +15,14 @@ fun <T : CharSequence?> test2(x: T): Int {
 inline fun <reified T : CharSequence?> test3(x: Any): Int {
   return when {
     x !is T -> 0
-    else -> x /*as CharSequence */.<get-length>()
+    else -> x /*as T */.<get-length>()
   }
 }
 
 inline fun <reified T : CharSequence> test4(x: Any?): Int {
   return when {
     x !is T -> 0
-    else -> x /*as CharSequence */.<get-length>()
+    else -> x /*as T */.<get-length>()
   }
 }