[Reassociation] Fix miscompile for va_arg arguments.

iisUnmovableInstruction() had a list of instructions hardcoded which are
considered unmovable. The list lacked (at least) an entry for the va_arg
and cmpxchg instructions.
Fix this by introducing a new Instruction::mayBeMemoryDependent()
instead of maintaining another instruction list.

Patch by Matthias Braun <matze@braunis.de>.

Differential Revision: http://reviews.llvm.org/D11577

rdar://problem/22118647

llvm-svn: 244244