Calls like read_leb<int32_t, true, true, true>(...) can be hard to read
and understand. This CL replaces the three boolean template parameters
with enums so that the call is read_leb<int32_t, kChecked, kAdvancePC, kTrace>(...)
now.

R=clemensh@chromium.org

Bug: v8:6921
Change-Id: Id876a727d5e17df721444e7e5a117ad5395071aa
Reviewed-on: https://chromium-review.googlesource.com/718204
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48607}