RyuJIT x86: allow long-typed struct fields to be recursively promoted

[pgavlin]

RyuJIT is currently unable to recursively promote fields of promoted struct vars. When targeting x86, this restriction causes representational issues for structs that have long-typed fields: normally, we would promote the variables that represent these fields, allowing them to be enregistered. Because we cannot perform the necessary recursive promotion, the variables that represent these long-typed fields are forced onto the stack.

category:cq
theme:structs
skill-level:expert
cost:medium
impact:medium