D3D11: fix constant buffer cache lookup. The code was accessing the constant buffer cache by the buffer index, but storing it by the register index. The fix is to use the register index in both cases. Bug: angleproject:7304 Change-Id: I750ba2911140660b42db8e0035984da5e94b14e4 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3668840 Commit-Queue: Stephen White <senorblanco@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>