Merge pull request dotnet/coreclr#6092 from sivarv/lsrawork

Lower, LSRA and Codegen support for reg optional operands

Commit migrated from https://github.com/dotnet/coreclr/commit/e9f06e041b379f3919455ca005681a9fcac72ea6

diff --cc src/coreclr/src/jit/codegenxarch.cpp
Simple merge

diff --cc src/coreclr/src/jit/gentree.h
Simple merge

diff --cc src/coreclr/src/jit/lowerxarch.cpp
Simple merge