[wasm] Cleanup wasm asm builtin wrappers

Split small chunks of assembly instructions into separate functions.
This makes the code easier to follow and to maintain, especially for
register allocation.

Drive-by: simplify stack-switching test.

R=ahaas@chromium.org

Bug: v8:12191
Change-Id: Id7544a3b2d16085540d9f1863a0eabd1f72f22bb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3461929Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79105}

test/mjsunit/wasm/stack-switching.js

@@ -27,7 +27,7 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
       .addBody([]).exportFunc();
   let suspender = new WebAssembly.Suspender();
   function js_import(i) {
-    return new Promise((resolve) => { resolve(42); });
+    return Promise.resolve(42);
   }
 
   // Wrap the import, instantiate the module, and wrap the export.
@@ -97,7 +97,7 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
       ]).exportFunc();
   let suspender = new WebAssembly.Suspender();
   function js_import() {
-    return new Promise((resolve) => { resolve(42); });
+    return Promise.resolve(42);
   };
   let wasm_js_import = new WebAssembly.Function(
       {parameters: [], results: ['externref']}, js_import);
@@ -143,7 +143,7 @@ load("test/mjsunit/wasm/wasm-module-builder.js");
   let i = 0;
   // The n-th call to the import returns a promise that resolves to n.
   function js_import() {
-    return new Promise((resolve) => { resolve(++i); });
+    return Promise.resolve(++i);
   };
   let wasm_js_import = new WebAssembly.Function(
       {parameters: [], results: ['externref']}, js_import);