[Win/x64] Update preserve_most to treat XMM registers like C (#73866)

As [scottmcm
described](https://discourse.llvm.org/t/conv-c-and-conv-preservemost-mix-badly-on-windows-x64/73054),
the `preserve_most` calling convention, as currently implemented, is a
bad fit for Windows on x64. The intent of `preserve_most` is "to make
the code in the caller as unintrusive as possible", but `preserve_most`
causes the caller to spill and restore ten SIMD registers. It would be
preferable to make `preserve_most` treat the XMM registers however the C
calling convention does on the target operating system.

This is a breaking change, but the documentation indicates that
`preserve_most` is still experimental, so I believe that ABI
compatibility is not yet a requirement.