[FIR2IR] Don't generate excess IR block from FirSingleExpressionBlock

This is needed to unify K1 and K2 behavior.

#KT-65064
#KT-63781 Fixed

compiler/testData/ir/irText/expressions/ifWithAssignment.sig.kt.txt

@@ -0,0 +1,5 @@
+// CHECK:
+//   Mangled name: #topLevelMethod(){}
+//   Public signature: /topLevelMethod|3993481242130118189[0]
+//   Public signature debug description: topLevelMethod(){}
+fun topLevelMethod(): Unit