This reverts commit baecb7d5.

Reason for revert: Broke Skia Win/ARM builder. See issue for details.

Original change's description:
> Refactor BitCount
> 
> POPCNT intrinsics cannot be used without hardware support, so a CPUID
> check and polyfills are required for some CPUs when using MSVC to
> avoid runtime failure.
> 
> Other changes include:
> - Clang: use builtins on all platforms to provide exact intent to the
> compiler;
> - MSVC on ARM: use dedicated intrinsics;
> - x86/x64 fallback is now branchless and works in constant time.
> 
> Bug: angleproject:4462
> Change-Id: I00fcabda1c842677d8cb4bfd280d932d0d10c0a5
> Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2102811
> Reviewed-by: Jamie Madill <jmadill@chromium.org>
> Reviewed-by: Geoff Lang <geofflang@chromium.org>
> Commit-Queue: Geoff Lang <geofflang@chromium.org>

TBR=geofflang@chromium.org,jmadill@chromium.org,lexa.knyazev@gmail.com

Change-Id: Ia1756abdf7da2aa9574149eb388915f97758bba0
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: angleproject:4462
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2112276Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>