When finding call-site argument locations, don't consider registers to be
location candidates if they will be clobbered between the copy to/from them
and call site. Doing so would present overwritten register values as entry
values in called functions.
This patch adds a collection of register units defined as we walk back from
the call site, and prevents the acceptance of a call-site parameter
location if it will be clobbered on that path.
Fixes https://github.com/llvm/llvm-project/issues/57444
Differential Revision: https://reviews.llvm.org/D141279
b2cf024280