Fix constexpr on clang

clang correctly notes that this branch is impossible to hit on 32 bit
systems, so wrap it in an if contexpr check to check for 32 bit, and
avoid the next branch entirely.

Tested: code compiles further on clang.  Unit tests pass.

Signed-off-by: Ed Tanous <edtanous@google.com>
Change-Id: Iccaab8402d839faa7c3f7cea457ef6bcba832f67