Speed up two functions in gpu_fusible.cc.
Speed up two functions in gpu_fusible.cc.

In SharedMemoryUsage() and NumUnnestedReductions(), iterate over
fused_instructions_computation()->instructions() instead of
MakeInstructionsPostorder().

Making the postorder list is expensive, and in these cases we don't care about
the order of iteration.

This is a 1.5x speedup to HLO passes on the particular model I'm benchmarking.

PiperOrigin-RevId: 394383751
Change-Id: Ib762da86a91b7a4647e3cff836324c7b00c6c380

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