[X86] Fix callee side of receiving byval args on the stack

See the discussion in
https://discourse.llvm.org/t/generic-llvm-ir-windows-x64-argument-passing-issue-in-llvm-11-0-0-and-later/71350

D51842 implemented byval lowering for Win64. D83175 made the call
lowering honor the "from now on treat this as a regular pointer" comment
also when the argument gets passed on the stack. However, it didn't
update the callee side.

Differential revision: https://reviews.llvm.org/D153020