Add an ensureMaxAlignment() function to MachineFrameInfo (analogous to

Add an ensureMaxAlignment() function to MachineFrameInfo (analogous to
ensureAlignment() in MachineFunction).  Also, drop setMaxAlignment() in
favor of this new function.  This creates a main entry point to setting
MaxAlignment, which will be helpful for future work.  No functionality
change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@158758 91177308-0d34-0410-b5e6-96231b3b80d8