The line being modified currently causes a 32-bit build with MSVC to
give out this warning:

C2397: conversion from 'int32_t' to 'v8::internal::byte' requires a
narrowing conversion.

Avoid the warning by declaring `shift` as type byte to start with.

Change-Id: Ib11c8e24811bfc6fe076b845be140e86b7ca38c5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2877949Reviewed-by: Zhi An Ng <zhin@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74451}