Unix: Unecessary struct copy while passing by value on stack

[sivarv]

Say caller foo() is passing a struct 's' by value on stack to bar(), at high-level code generated is

// Make a copy of s by assigning to a local t on foo()'s stack frame
t = s
copy t on to stack
bar()

t = s copy is unnecessary as 's' could be directly copied to stack.

[sejongoh]

dotnet/coreclr#6037 is merged.

dotnet/coreclr#6266, dotnet/coreclr#6265, and dotnet/coreclr#6264 are opened for cases not handled by dotnet/coreclr#6037.

[CarolEidt]

Closing this, as dotnet/coreclr#18358 addressed the extra copies when passed on stack. dotnet/coreclr#6264 remains open because it includes extra copies when passed in registers.