[WASM] Fold redundant patterns with drop instruction on a wasm level

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/Operators.kt

@@ -367,6 +367,9 @@ enum class WasmOp(
     REF_CAST("ref.cast", 0xFB_41),
 
     BR_ON_CAST("br_on_cast", 0xFB_42, listOf(LABEL_IDX)),
+
+    // Pseudo-instruction, just alias for a normal call. It's used to easily spot get_unit on the wasm level.
+    GET_UNIT("call", 0x10, FUNC_IDX)
     ;
 
     constructor(mnemonic: String, opcode: Int, vararg immediates: WasmImmediateKind) : this(mnemonic, opcode, immediates.toList())

wasm/wasm.ir/src/org/jetbrains/kotlin/wasm/ir/Utils.kt

@@ -42,6 +42,16 @@ fun foldWasmInstructions(prev: WasmInstr?, next: WasmInstr): List<WasmInstr>? {
     if (next.operator == WasmOp.UNREACHABLE && prev.operator in listOf(WasmOp.UNREACHABLE, WasmOp.RETURN))
         return listOf(prev)
 
+    if (next.operator == WasmOp.DROP) {
+        // simple pure instruction + drop -> nothing
+        if (prev.operator == WasmOp.GET_UNIT || prev.operator == WasmOp.REF_NULL)
+            return listOf()
+
+        // return + drop -> return
+        if (prev.operator == WasmOp.RETURN)
+            return listOf(prev)
+    }
+
     return null
 }