Revert "Reuse %rax after calling __chkstk on win64"

This reverts commit 01f8d579f7672872324208ac5bc4ac311e81b22e.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185781 91177308-0d34-0410-b5e6-96231b3b80d8