Revert of [wasm] Update {i32,i64}.const to use signed leb128

src/wasm/ast-decoder.h

Index: src/wasm/ast-decoder.h
diff --git a/src/wasm/ast-decoder.h b/src/wasm/ast-decoder.h
index d173986e72c1fd86e80d8b18e030b000539c33ee..465bacaab84918b21a1db437b37353940bfb25bc 100644
--- a/src/wasm/ast-decoder.h
+++ b/src/wasm/ast-decoder.h
@@ -46,8 +46,8 @@
   int32_t value;
   int length;
   inline ImmI32Operand(Decoder* decoder, const byte* pc) {
-    value =
-        bit_cast<int32_t>(decoder->checked_read_i32v(pc, 1, &length, "immi32"));
+    value = bit_cast<int32_t>(decoder->checked_read_u32(pc, 1, "immi32"));
+    length = 4;
   }
 };
 
@@ -55,8 +55,8 @@
   int64_t value;
   int length;
   inline ImmI64Operand(Decoder* decoder, const byte* pc) {
-    value =
-        bit_cast<int64_t>(decoder->checked_read_i64v(pc, 1, &length, "immi64"));
+    value = bit_cast<int64_t>(decoder->checked_read_u64(pc, 1, "immi64"));
+    length = 8;
   }
 };