Gas loop optimization in MerkleProof.sol

[CarsonCase]

Closes #5028 by caching the length of the proof to prevent an unnecessary DUP call (3 gas per iteration)

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[cairoeth]

@CarsonCase reverted back the unchecked block because it's done automatically by Solidity 0.8.22 onwards, and also made the i variable explicit (per the style guide)

[cairoeth]

Gas difference:

	Normal	Optimizer (1000 runs)	Via IR
testProcessProof (unoptimized)	μ: 41703, ~: 41638	μ: 41699, ~: 41638	μ: 34632, ~: 34667
testProcessProof (optimized)	μ: 41335, ~: 41274	μ: 41331, ~: 41274	μ: 37574, ~: 37602
diff	μ: -368, ~: -364	μ: -368, ~: -364	μ: +2,942, ~: +2,935
testprocessProofCalldata (unoptimized)	μ: 31410, ~: 31343	μ: 31362, ~: 31299	μ: 24572, ~: 24567
testprocessProofCalldata (optimized)	μ: 31418, ~: 31351	μ: 31370, ~: 31307	μ: 24572, ~: 24567
diff	μ: +8, ~: +8	μ: +8, ~: +8	μ: 0, ~: 0

[ernestognw]

It does look good to me but I am not sure if the tradeoff makes sense.

From the gas benchmarks we can see --via-ir is even worse in the case of memory. For the calldata one I see no difference, which is consistent with how --via-ir works as it can assume that calldata never changes, thus it can be taken out of the loop.

At the moment, seems like we're largely affecting projects using --via-ir but improving performance for those who don't, and I don't have an opinion yet 🤔

Not opposed to closing this yet, but I'd like to hear @Amxx take, perhaps he has more clarity on why via IR is getting worse results