RyuJIT: Allow promotions of structs with fields of struct containing a single primitive field.

[sivarv]

Right now the existing implementation allows only structs with pointer sized fields. This would mean int/uint/ref/byref on 32-bit and long/ulong/ref/byref on 64-bit. We should be able to permit promotion of a following struct on 64-bit, for example

struct Foo { struct Bar { int a }, int b}

Similarly the following struct both on 32-bit and 64-bit targets

struct Foo { struct Bar { short a }, short b, int c }
category:cq
theme:structs
skill-level:expert
cost:medium

[AndreyAkinshin]

@sivarv, probably related to https://github.com/dotnet/coreclr/issues/6839

[JosephTremoulet]

n.b. I think this was filed because it would be a smallish step implementation-wise, not to target a particular code sequence, so we'd probably want to pick this up as part of a bigger push to remove artificial limitations in our struct promotion (which we generally want to do because using structs to avoid heap allocation shouldn't cause code quality degradation), not for its own sake.

[AndreyAkinshin]

@JosephTremoulet,

so we'd probably want to pick this up as part of a bigger push to remove artificial limitations in our struct promotion

LGTM.

[jakobbotsch]

I believe we are able to do this today, they don't need to be pointer sized anymore.