Optimize montgomerySub()

[IAvecilla]

Context: EcPairing.yul#L314

Description: The implementation of the montgomerySub(a, b) fallbacks to montgomeryAdd(a, P-b). This however can be implemented more efficiently as:

• compute difference a - b,
• add back P if the difference overflows.

The change gives 4% gas cost reduction.

Recommendation: Change the implementation of montgomerySub() to:

diff --git a/precompiles/EcPairing.yul b/precompiles/EcPairing.yul
index f70813e..4e0c526 100644
--- a/precompiles/EcPairing.yul
+++ b/precompiles/EcPairing.yul
@@ -319,7 +319,10 @@ object "EcPairing" {
             /// @param subtrahend The subtrahend in Montgomery form.
             /// @return ret The result of the Montgomery addition.
             function montgomerySub(minuend, subtrahend) -ret {
-                ret := montgomeryAdd(minuend, sub(P(), subtrahend))
+                ret := sub(minuend, subtrahend)
+                if lt(minuend, subtrahend) {
+                    ret := add(ret, P())
+                }
             }

             /// @notice Computes the Montgomery multiplication using the Montgomery reduction algorithm (REDC).

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