Unix: Unnecessary copies for promoted struct arguments

[sejongoh]

If an struct argument is promoted to registers, the codegen cannot handle this and copies should be generated in fgMorphArgs. It is desired to avoid copies and make the codegen copy struct directly to outgoing argument area.

Disabling the following code in `fgMorphArgs' can reproduce the problem.

if (lclVar != NULL)
{
    // If the struct is promoted to registers, it has to be materialized
    // on stack. We may want to support promoted structures in
    // codegening pugarg_stk instead of creating a copy here.
    LclVarDsc*  varDsc = &lvaTable[lclVar->gtLclVarCommon.gtLclNum];
    needCpyBlk = varDsc->lvPromoted;
}

[sejongoh]

Original issue: #5024

[CarolEidt]

Closed with dotnet/coreclr#18358