[X86][SSE] Begun adding modulo rotate support to LowerRotate

Prep work for PR38243 - mainly adding comments on where we need to add modulo support (doing so at the moment causes massive codegen regressions).

I've also consistently added support for modulo folding for uniform constants (although at the moment we have no way to trigger this) and removed the old assertions.

llvm-svn: 348366