Ignore load/store until stack address computation

No longer conservatively assume a load/store accesses the stack when we
can prove that we did not compute any stack-relative address up to this
point in the program.

We do this in a cheap not-quite-a-dataflow-analysis: Assume
`NoStackAddressUsed` when all predecessors of a block already guarantee
it. Process blocks in reverse post order to guarantee that except for
loop headers we have processed all predecessors of a block before
processing the block itself. For loops we accept the conservative answer
as they are unlikely to be shrink-wrappable anyway.

Differential Revision: https://reviews.llvm.org/D152213