Consolidate implementation of Rotate and PopCount

[grant-d]

RotateLeft, RotateRight

There are several implementations of RotateLeft and RotateRight in the stack.

• This PR consolidates them into a central class
• The actual implementations are unchanged, ie uses the same standard idioms from original callsites
• All BitOps methods pass units

Unlike the related PRs (LZCNT, TZCNT) where the goal was to fix & optimize the code,
the benefits here are:

• Less proliferation of identical helpers
• Existing code/callsites are more self-documenting
• New code can use these new pseudo-intrinsics
• Dependency/profile analysis

Note related JIT optimization which makes all such methods pseudo-intrinsic: #1830

PopCount

For PopCount, the goal is to:

• Centralize and optimize existing (mostly downstream) callsites (eg1, eg2)
• Provide optimized utility methods for new code to use

TrailingZeroCount

• Perf tweak in TrailingZeroCount related to MSIL generation (removing redundant conv.u8)

cc @tannergooding

[grant-d]

@dotnet-bot test this please

[grant-d]

@dotnet-bot test coreclr-ci (Build Linux arm debug) please
@dotnet-bot test coreclr-ci (Build Linux arm release) please

[grant-d]

/azp run coreclr-ci

[grant-d]

/azp run

[grant-d]

Lots of infra failures
Not sure how to get the azp tasks to restart.
Per https://github.com/dotnet/corefx/issues/35121#issuecomment-462849658 my commands should work, maybe I don't have permissions?
cc @safern

[safern]

Based on my conversation with Azure Devops folks, you have to be collaborator in order to be able to trigger builds from comments, I think contributors can't do it yet 😢

[safern]

/azp run coreclr-ci

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[grant-d]

@dotnet-bot test OSX10.12 x64 Checked Innerloop Build and Test please
@dotnet-bot test Windows_NT x64 Release CoreFX Tests please

[jkotas]

@safern Are you able to make sense of all the failing Azure Pipelines tasks on this PR? Are they all infrastructure issues?

[grant-d]

@dotnet-bot test Windows_NT x64 Release CoreFX Tests please

[grant-d]

This last check is failing on a ping test, which sounds like infra. Hopefully it goes through this time

[jkotas]

Thanks!

[grant-d]

Thanks again @jkotas, once again the feedback is appreciated. Always something new to learn here.