Fix false USELESS_CAST in function with expression body

Note that current behaviour is made similar to the case with
properties initializers/accessors, which means that more complex
cases are not covered yet (see KT-20801) #KT-20802 fixed.

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

@@ -0,0 +1,12 @@
+fun <T> id(x: T): T = x
+
+fun foo() = 1 as Any
+fun bar() = id(1) as Any
+fun baz() = (1 + 1) as Any
+
+val functionLiteral1 = fun() = 1 as Any
+val functionLiteral2 = fun() = id(1) as Any
+val functionLiteral3 = fun() = (1 + 1) as Any
+
+// TODO: this and more complex cases are not supported yet
+fun baz(b: Boolean) = if (b) 1 <!USELESS_CAST!>as Any<!> else 42 <!USELESS_CAST!>as Any?<!>