Moved the hand-rolled implementation to Subzero.
As we've started exposing bit intrinsics, we might as fix the TODOs.

Bug: b/126873455
Tests: dEQP-VK.glsl.builtin.function.integer.bitcount.*
Change-Id: Ic37dfd5d73187f2b3afa444abfd9e22439c871b1
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/28792
Presubmit-Ready: Ben Clayton <bclayton@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>
Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>