[Arm64] Fix genCodeForCpObj()

[Arm64] Fix genCodeForCpObj()

CORINFO_HELP_ASSIGN_BYREF is the functional equivalent to

   *dst = *src;
   ++dst;
   ++src;

Note: Description ignores barriers.

Since src and dst are incremented the registers are marked dead.

In genCodeForCpObj(), the incremented values are will be reused
and therefore must be treated as live.  Add code to explicitly
force them live for this use case.

Commit migrated from https://github.com/dotnet/coreclr/commit/a5d464837248bae64845fb3e8207821d29f316d8