[FIR] Remove the fast path if as insignificant

This if is a bit inconsistent with the
LUB checking approach, so if it doesn't
improve performance it's better to remove it.

compiler/testData/diagnostics/tests/BinaryCallsOnNullableValues.fir.kt

@@ -10,8 +10,8 @@ fun f(): Unit {
     x <!UNSAFE_OPERATOR_CALL!><<!> 1
     <!ASSIGNMENT_TYPE_MISMATCH!>x += 1<!>
 
-    <!EQUALITY_NOT_APPLICABLE_WARNING!>x == 1<!>
-    <!EQUALITY_NOT_APPLICABLE_WARNING!>x != 1<!>
+    x == 1
+    x != 1
 
     <!EQUALITY_NOT_APPLICABLE!>A() == 1<!>