[StatepointLowering] Update StatepointMaxSlotsRequired correctly

Now that we don't always add an element to AllocatedStackSlots if we
don't find a pre-existing unallocated stack slot, bumping
StatepointMaxSlotsRequired to `NumSlots + 1` is not correct.  Instead
bump the statistic near the push_back, to
Builder.FuncInfo.StatepointStackSlots.size().

llvm-svn: 261348