[FIR] Remove redundant checks from isUpcast() and isExactTypeCast()

`T.() -> R` and `(T) -> R` should only be
different for resolution, but casts are only
about typechecking. Values of either of
these types can always be put into variables
of the other type.

compiler/testData/diagnostics/tests/cast/ExtensionAsNonExtension.kt

@@ -1,4 +1,3 @@
-// FIR_IDENTICAL
 fun f(a: (Int) -> Unit) {
     a as Int.() -> Unit