Added the missing implementation for isinf and isnan.

Change-Id: Ice603956a4ed8ef9515ae8f501b2876ce5fab584
Reviewed-on: https://swiftshader-review.googlesource.com/13608Tested-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>