JIT: Insert writebacks more eagerly (#87869)

JIT: Insert writebacks more eagerly (#87869)

Before starting replacements within a statement we now look for
physically promoted struct locals passed as call args and write those
back if necessary. This has two benefits:
1. It avoids creating a lot of comma nodes that we would otherwise
   create
2. More importantly, it allows morph's last-use copy omission to kick in
   because we no longer have complex trees as the argument.

Fix #87471