JIT: add struct promotion stress mode

[JosephTremoulet]

Given the issues with dotnet/corefx#20025, #8185, and #8195, which were triggered by dotnet/coreclr#10453 even though it passed normal core and desktop testing, it may well be a good idea to add a stress mode to try to flush out such bugs. The stress mode would short-circuit this check so it never decides to undo a promotion.

Going further, short-circuiting lvaShouldPromoteStructVar to always return true as part of struct promotion stress mode could help flush out more issues.

category:correctness
theme:structs
skill-level:intermediate
cost:medium
impact:small

[AndyAyersMS]

Have bits of this laying around, I should finish it one day.

[jakobbotsch]

We have struct promotion stress modes for physical promotion, and it's unlikely we will do anything more for old promotion.