Map::kBitFieldOffset should be loaded as a byte data.  This patch
fixes the loading instruction of Map::kBitFieldOffset in lazy
accessors.

Bug: v8:6795, v8:6156
Change-Id: I8fbc88ed44fb43a24335fc81f75b7199ca80212c
Reviewed-on: https://chromium-review.googlesource.com/656862
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47918}