[X86] Handle EAX being live when calling chkstk for x86_64

EAX can turn out to be alive here, when shrink wrapping is done
(which is allowed when using dwarf exceptions, contrary to the
normal case with WinCFI).

This fixes PR36487.

Differential Revision: https://reviews.llvm.org/D43968

llvm-svn: 326764