Revert [X86] When using Win64 ABI, exit with error if SSE is disabled for varargs

It unnecessarily breaks previously-working code which used varargs,
but didn't pass any float/double arguments (such as EDK2).

Also revert the fixup on top of that:
Revert [X86] Fix a test from r357317

This reverts r357317 (git commit d413f41d)
This reverts r357380 (git commit 7af32444)

llvm-svn: 357718