Switch statements were resetting the 'enable' execution mask stack to
all lanes, which caused us to execute the switch operations even for
lanes that were already done looping.

Change-Id: Ie5a525e8de8d5e61d4c61bcee81bd81fa862d917
Reviewed-on: https://swiftshader-review.googlesource.com/15828Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>