Merge pull request dotnet/coreclr#4273 from sivarv/multireg2

Force IR to be always var=call in case of multi-reg returning call nodes plus refactoring changes

Commit migrated from https://github.com/dotnet/coreclr/commit/5ef243f597b8198efce1add587f16aae59b1f568

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/importer.cpp
Simple merge