Using a much more compact input layout structure allows us to save
quite a bit of time comparing input layouts, or computing hashes.
A subsequent patch shrinks the size of the structure further.

BUG=angleproject:959

Change-Id: I7a0f10428cc7e96967e60cc085ff2e8779718978
Reviewed-on: https://chromium-review.googlesource.com/277288Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>