FIR: Change the Fir2Ir handling of smart casts.

Generate the expression with the original type and then insert
an implicit conversion. That matches the behavior of psi2ir
better and therefore avoids breaking backend assumptions.

In particular, IrGetValue expects the type of the underlying
symbol and the type of the IrGetValue to be the same.

compiler/testData/codegen/box/casts/functions/asFunKSmall.kt

@@ -1,3 +1,7 @@
+// FIR inserts incorrect smart casts based on the cast in the
+// lambda. However, that cast can fail and the failure can be caught
+// by the inline function.
+// IGNORE_BACKEND_FIR: JVM_IR
 // IGNORE_BACKEND: JS_IR
 // TODO: muted automatically, investigate should it be ran for JS or not
 // IGNORE_BACKEND: JS, NATIVE