[X86] Clean up the tail call eligibility logic

Summary:
The logic here isn't straightforward because our support for
TargetOptions::GuaranteedTailCallOpt.

Also fix a bug where we were allowing tail calls to cdecl functions from
fastcall and vectorcall functions. We were special casing thiscall and
stdcall callers rather than checking for any convention that requires
clearing stack arguments before returning.

Reviewers: hans

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D14024

llvm-svn: 251137