Work around a GCC miscompilation issue.

Bug 18470793

Change-Id: Idac85c738810db112e7af56f5e1635c2ac6d325c
Reviewed-on: https://swiftshader-review.googlesource.com/1841
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <capn@google.com>
Tested-by: Nicolas Capens <capn@google.com>