The range checks in InputAt and ReplaceInput are relatively expensive
given the high frequency of these calls and the bitfield decoding
required for InputCount. Make these DCHECKs instead of CHECKS

BUG=v8:9684

Change-Id: Ibee0f9b06b4581f3bc1176d1fe57de33cdd73cdc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2735274Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Ross McIlroy <rmcilroy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#73249}