[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.kt

@@ -0,0 +1,28 @@
+fun topLevelMethod() {
+    var z = 1
+    if(true) { z = 2 } else { z = 3 }
+    if(true)   z = 4   else   z = 5
+
+    if(true) { z += 6 } else { z += 7 }
+    if(true)   z += 8   else   z += 9
+
+    when(z) {
+        1 -> { z = 10 }
+        else -> { z = 11 }
+    }
+
+    when(z) {
+        1 -> z = 12
+        else -> z = 13
+    }
+
+    when(z) {
+        1 -> { z += 14 }
+        else -> { z += 15 }
+    }
+
+    when(z) {
+        1 -> z += 16
+        else -> z += 17
+    }
+}