Avoid reserving an ARM base pointer during register allocation.

Once register allocation has started the reserved registers are frozen.

Fix the ARM canRealignStack() hook to respect the frozen register state.
Now the hook returns false if register allocation was started with frame
pointer elimination enabled.

It also returns false if register allocation started without a reserved
base pointer, and stack realignment would require a base pointer.  This
bug was breaking oggenc on armv6.

No test case, an upcoming patch will use this functionality to realign
the stack for spill slots when possible.

llvm-svn: 147578