[Target/X86] Don't use callee-saved registers in a Win64 tail call on non-Windows.

Summary:
A small bit that I missed when I updated the X86 backend to account for
the Win64 calling convention on non-Windows. Now we don't use dead
non-volatile registers when emitting a Win64 indirect tail call on
non-Windows.

Should fix PR23710.

Test Plan: Added test for the correct behavior based on the case I posted to PR23710.

Reviewers: rnk

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10258

llvm-svn: 239111