Add a StackSlots::layout() method which computes the total stack frame
size and assigns offsets to all spill slots and local variables so they
don't interfere with each other or with incoming or outgoing function
arguments.
Stack slots are given an ad hoc alignment that is the natural alignment
for power-of-two sized spill slots, up to the stack pointer alignment.
It is possible we need explicit stack slot alignment in the future, but
at least for spill slots, this scheme is likely to work for most ISAs.
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