Move some LEGACY_BACKEND code from codegencommon.cpp to codegenlegacy.cpp

Move some LEGACY_BACKEND code from codegencommon.cpp to codegenlegacy.cpp

And, move some header file changes from codegen.h to codegenclassic.h.

The code moved were whole functions with no apparent tight coupling with
other code already in codegencommon.cpp.

The code moved had some #ifndef LEGACY_BACKEND portions, no longer
needed, removed.

Commit migrated from https://github.com/dotnet/coreclr/commit/fa74a694fe525847e8833613e2b31edc813b30b3