Pulling out x86 decomposition code

[sejongoh]

As discussed in dotnet/coreclr#5380, pulling out the decomposition code is suggested to prevent the lowering class and file from being huge and complicated.

[sejongoh]

I assigned this to Bruce, so that he can take a look about the idea.

[BruceForstall]

@sejongoh @mikedn @adiaaida @CarolEidt I'm planning to refactor x86 long decomposition code into its own file and class, per this issue request. Seems like a very good idea. It will cause nasty manual merge issues for anyone working on this code, however.

[michellemcdaniel]

Thanks for the heads up.