Fix implicit fallthrough warnings treated as errors

Fix implicit fallthrough warnings treated as errors

Ideally we'd use the [[fallthrough]] attribute to silence this, but that
isn't supported until C++17 and we're sticking with C++11 for now.

Bug b/120382288

Change-Id: I98f28e42505ed496b0b35c29a7dc03fca18cf4ff
Reviewed-on: https://swiftshader-review.googlesource.com/c/23128
Reviewed-by: Greg Hartman <ghartman@google.com>
Reviewed-by: Chris Forbes <chrisforbes@google.com>
Tested-by: Nicolas Capens <nicolascapens@google.com>