Avoid marking tmp as DoNotEnregister in tmp=GT_CALL() where call returns a enregisterable struct in two return registers

[sivarv]

Right now codegenxarch.cpp is implemented with the assumption that the tmp to which struct returned in two registers is assigned does not live in a register. For example, when a call returns Vector3/4, we will have a tmp = call(), where tmp is of SIMD12/16 type and can be allocated a register. In this case it will lead to better code quality if codegen directly generates reg-to-reg moves instead of save to a temp on stack.

category:cq
theme:calling-convention
skill-level:intermediate
cost:small

[CarolEidt]

A repro, in particular one that reflects an actual real-world scenario, would be useful in prioritizing this.

[CarolEidt]

Fixed by #36862