Switch statements and loops should not reset the execution enable mask
to all lanes at the end, because they can be nested. Instead we can
just restore them to the mask value before the switch/loop.

Change-Id: I4089cbf637c28f39a536de1e68e2e513ead1ec7f
Reviewed-on: https://swiftshader-review.googlesource.com/15908Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>