Ctlz and Cttz are stubbed for now. Will fix after the builds turn green.

Bug: b/130343040
Tests: dEQP-VK.glsl.builtin.precision.*
Tests: dEQP-VK.glsl.builtin.function.integer.findMSB.*
Change-Id: Ibff6a52bee7f5440c536dfc7a61ad4ba3e96743d
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28868Tested-by: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>