[X86] Correctly select registers to pop into for x86_64

When trying to replace an add to esp with pops, we need to choose dead
registers to pop into. Registers clobbered by the call and not imp-def'd
by it should be safe. Except that it's not enough to check the register
itself isn't defined, we also need to make sure no overlapping registers
are defined either.

This fixes PR26711.

Differential Revision: http://reviews.llvm.org/D18029

llvm-svn: 263139