The CASE_BODY macro is inconvenient for debugging, as gdb points only at
the SINGLE_CASE line, not the actual expanded line.

Converting it into a templatized function should preserve optimization
opportunities for the compiler while making debugging much easier.

Bug: v8:6624
Change-Id: I864eff190e39e3230c529ced5c4919aa875763b1
Reviewed-on: https://chromium-review.googlesource.com/612084
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47471}