And if the format we picked has extra bits, try to clear it instead
of load the value. Don't store these extra bits as values as well.

Bug: angleproject:4459
Change-Id: If5d0e31aca1453deab970d0dbcf8886a5e6ed51c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2095850
Commit-Queue: Charlie Lao <cclao@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>