[wasm][test] Add method to create signed LEB values

This method will be used for a test with multiple code spaces, to
encode large function indexes. The current implementation in
{wasmI32Const} just always uses 5 bytes for encoding the LEB value.
This CL adds a {wasmSignedLeb} function which properly encodes the
value, and adds tests for that.

Drive-by: Clean up the rest of {test-wasm-module-builder.js}.

R=mstarzinger@chromium.org

Bug: v8:9477
Change-Id: Ide2d90eed9d40aa28df680fbb413275346d9c0b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1725623Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62990}

test/mjsunit/wasm/wasm-module-builder.js

@@ -552,7 +552,7 @@ class Binary {
     this.buffer[this.length++] = val >> 24;
   }
 
-  emit_leb(val, max_len) {
+  emit_leb_u(val, max_len) {
     this.ensure_space(max_len);
     for (let i = 0; i < max_len; ++i) {
       let v = val & 0xff;
@@ -567,11 +567,11 @@ class Binary {
   }
 
   emit_u32v(val) {
-    this.emit_leb(val, kMaxVarInt32Size);
+    this.emit_leb_u(val, kMaxVarInt32Size);
   }
 
   emit_u64v(val) {
-    this.emit_leb(val, kMaxVarInt64Size);
+    this.emit_leb_u(val, kMaxVarInt64Size);
   }
 
   emit_bytes(data) {
@@ -1384,13 +1384,24 @@ class WasmModuleBuilder {
   }
 }
 
-function wasmI32Const(val) {
-  let bytes = [kExprI32Const];
-  for (let i = 0; i < 4; ++i) {
-    bytes.push(0x80 | ((val >> (7 * i)) & 0x7f));
+function wasmSignedLeb(val, max_len = 5) {
+  let res = [];
+  for (let i = 0; i < max_len; ++i) {
+    let v = val & 0x7f;
+    // If {v} sign-extended from 7 to 32 bits is equal to val, we are done.
+    if (((v << 25) >> 25) == val) {
+      res.push(v);
+      return res;
+    }
+    res.push(v | 0x80);
+    val = val >> 7;
   }
-  bytes.push((val >> (7 * 4)) & 0x7f);
-  return bytes;
+  throw new Error(
+      'Leb value <' + val + '> exceeds maximum length of ' + max_len);
+}
+
+function wasmI32Const(val) {
+  return [kExprI32Const, ...wasmSignedLeb(val, 5)];
 }
 
 function wasmF32Const(f) {