I/O blocks can specify location for each member of the block separately,
in arbitrary fashion.  This change fixes up the code that validates
varying locations to take this into account.

Bug: angleproject:3580
Change-Id: If883347fc5db9f18722e41938d1b61fa64650d0c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2578047Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>