[X86] Fix tailcall return address clobber bug.

[X86] Fix tailcall return address clobber bug.

This bug (llvm.org/PR28124) was introduced by r237977, which refactored
the tail call  sequence to be generated in two passes instead of one.

Unfortunately, the stack adjustment produced by the first pass was not
recognized by X86FrameLowering::mergeSPUpdates() in all cases, causing
code such as the following, which clobbers the return address, to be
generated:

popl    %edi
popl    %edi
pushl   %eax
jmp     tailcallee              # TAILCALL

To fix the problem, the entire stack adjustment is performed in
X86ExpandPseudo::ExpandMI() for tail calls.

Patch by Magnus Lång <margnus1@gmail.com>

Differential Revision: http://reviews.llvm.org/D21325

llvm-svn: 275103