[FIR] Remove or replace whole expressions in FIR diagnostic messages

FIR expressions rendered by FirRenderer don't look very nice in error
messages anyway, and additionally, they can become arbitrarily large,
so we shouldn't use them in messages.

#KT-59449 Fixed

compiler/testData/diagnostics/tests/senselessComparison/complexExpression.fir.kt

@@ -0,0 +1,7 @@
+// RENDER_DIAGNOSTICS_FULL_TEXT
+
+fun String?.repro(): Boolean {
+    return <!SENSELESS_COMPARISON!>this?.let {
+        return false
+    } == true<!>
+}