Merge pull request dotnet/coreclr#4042 from sivarv/refactor2

Refactor Unix structDesc of GenTreeCall node into MultiRegReturnTypeDesc

Commit migrated from https://github.com/dotnet/coreclr/commit/d62613deb7af2c1d9126e3abfa368536d3df5a04

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

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

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

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

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