[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.txt

@@ -0,0 +1,64 @@
+fun topLevelMethod() {
+  var z: Int = 1
+  when {
+    true -> { // BLOCK
+      z = 2
+    }
+    else -> { // BLOCK
+      z = 3
+    }
+  }
+  when {
+    true -> z = 4
+    else -> z = 5
+  }
+  when {
+    true -> { // BLOCK
+      z = z.plus(other = 6)
+    }
+    else -> { // BLOCK
+      z = z.plus(other = 7)
+    }
+  }
+  when {
+    true -> z = z.plus(other = 8)
+    else -> z = z.plus(other = 9)
+  }
+  { // BLOCK
+    val tmp_0: Int = z
+    when {
+      EQEQ(arg0 = tmp_0, arg1 = 1) -> { // BLOCK
+        z = 10
+      }
+      else -> { // BLOCK
+        z = 11
+      }
+    }
+  }
+  { // BLOCK
+    val tmp_1: Int = z
+    when {
+      EQEQ(arg0 = tmp_1, arg1 = 1) -> z = 12
+      else -> z = 13
+    }
+  }
+  { // BLOCK
+    val tmp_2: Int = z
+    when {
+      EQEQ(arg0 = tmp_2, arg1 = 1) -> { // BLOCK
+        z = z.plus(other = 14)
+      }
+      else -> { // BLOCK
+        z = z.plus(other = 15)
+      }
+    }
+  }
+  { // BLOCK
+    val tmp_3: Int = z
+    when {
+      EQEQ(arg0 = tmp_3, arg1 = 1) -> z = z.plus(other = 16)
+      else -> z = z.plus(other = 17)
+    }
+  }
+}
+