There's a AsyncBuiltinsAssembler::InitializeNativeClosure() helper which
is used by the `await` builtins to initialize the native closures, which
are registered as fulfill/reject handlers. This helper initializes a
JSFunction based on the builtin SharedFunctionInfo. Since we are dealing
with builtins here, there's no point in using the fully generic helper
CodeStubAssembler::GetSharedFunctionInfoCode(), but we can immediately
assume that the SharedFunctionInfo::function_data() field contains a Smi
builtin index (guarded by TNode CASTs).

This almost cuts the generated code size for the `await` builtins in a
half, and might also yield some performance improvements due to reduced
register and instruction cache pressure.

Bug: v8:7253, v8:8238
Change-Id: I3415c2f9e0f04a7154c4bf4c3fe8156854dbfe13
Reviewed-on: https://chromium-review.googlesource.com/c/1281604Reviewed-by: Sathya Gunasekaran <gsathya@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56662}