Stack Alignment: when creating stack objects in MachineFrameInfo, make sure

the alignment is clamped to TargetFrameLowering.getStackAlignment if the target
does not support stack realignment or the option "realign-stack" is off.

This will cause miscompile if the address is treated as aligned and add is
replaced with or in DAGCombine.

Added a bool StackRealignable to TargetFrameLowering to check whether stack
realignment is implemented for the target. Also added a bool RealignOption
to MachineFrameInfo to check whether the option "realign-stack" is on.

rdar://12713765

llvm-svn: 169197