Unix: Unnecessary struct copy while passsing struct of size <=16

[sejongoh]

Struct arguments whose size is <= 16 bytes generate unnecessary copies. @sivarv categorized the cases below:

The following are the struct passing cases on Amd64 Unix that are of interest:

1. struct of size 1,2,4 or 8 in memory/register is getting passed in an arg reg or on stack
2. struct of size 3,5,6,7 in memory/register is getting passed in an arg reg or on stack
3. struct of size 9-16 bytes in memory is getting passed in two arg registers
4. struct of size 9-16 bytes (e.g. SIMD structs Vector3 and Vector4) in a register is getting passed in two arg registers
5. struct of size 9-16 bytes in memory/register is getting passed on stack.

In fgMorphArgs, eeGetSystemVAmd64PassStructInRegisterDescriptor classifies the cases above as register argument except simd from vector<t> and sets structDesc.passedInRegisters to true, this always calls fgMakeOutgoingStructArgCopy() and generates copies.

[CarolEidt]

dotnet/coreclr#18358 eliminated the unnecessary copy when passed on stack.
There are still cases where structs are copied unnecessarily when passed in registers.

[erozenfeld]

For some reason it wasn't possible to update the description of this issue. I created a copy: dotnet/coreclr#19425.