[Wasm] Don't generate unnecessary unreachable instructions

Helps with WAT readability

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

@@ -9,6 +9,8 @@ abstract class WasmExpressionBuilder {
     abstract fun buildInstr(op: WasmOp, vararg immediates: WasmImmediate)
     abstract var numberOfNestedBlocks: Int
 
+    abstract val lastInstr: WasmOp?
+
     fun buildConstI32(value: Int) {
         buildInstr(WasmOp.I32_CONST, WasmImmediate.ConstI32(value))
     }
@@ -30,6 +32,10 @@ abstract class WasmExpressionBuilder {
     }
 
     fun buildUnreachable() {
+        // Unreachable is not needed
+        if (lastInstr == WasmOp.UNREACHABLE || lastInstr == WasmOp.RETURN)
+            return
+
         buildInstr(WasmOp.UNREACHABLE)
     }
 

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

@@ -19,4 +19,7 @@ class WasmIrExpressionBuilder(
             assert(value >= 0) { "end without matching block" }
             field = value
         }
+
+    override val lastInstr: WasmOp?
+        get() = expression.lastOrNull()?.operator
 }