The 'enable' execution mask for if/else, which is also used by loops,
was not being applied to predicate the instructions within a loop.

Bug b/28710510

Change-Id: Ibd8e962ef60de76ec1c018ca064a791abf214c94
Reviewed-on: https://swiftshader-review.googlesource.com/15948Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>