Reapply r146997, "Heed spill slot alignment on ARM."

Reapply r146997, "Heed spill slot alignment on ARM."

Now that canRealignStack() understands frozen reserved registers, it is
safe to use it for aligned spill instructions.

It will only return true if the registers reserved at the beginning of
register allocation allow for dynamic stack realignment.

<rdar://problem/10625436>

llvm-svn: 147579