chore: reuse stage+1 twiddle in DIF-8, drop redundant BigInt reads (#1627)

Author: phrwlk

backend/groth16/bls12-377/mpcsetup/lagrange.go

@@ -90,7 +90,6 @@ func kerDIF8G1(a []curve.G1Affine, twiddles [][]fr.Element, stage int) {
 	butterflyG1(&a[5], &a[7])
 	twiddles[stage+1][1].BigInt(&twiddle)
 	a[3].ScalarMultiplication(&a[3], &twiddle)
-	twiddles[stage+1][1].BigInt(&twiddle)
 	a[7].ScalarMultiplication(&a[7], &twiddle)
 	butterflyG1(&a[0], &a[1])
 	butterflyG1(&a[2], &a[3])
@@ -118,7 +117,6 @@ func kerDIF8G2(a []curve.G2Affine, twiddles [][]fr.Element, stage int) {
 	butterflyG2(&a[5], &a[7])
 	twiddles[stage+1][1].BigInt(&twiddle)
 	a[3].ScalarMultiplication(&a[3], &twiddle)
-	twiddles[stage+1][1].BigInt(&twiddle)
 	a[7].ScalarMultiplication(&a[7], &twiddle)
 	butterflyG2(&a[0], &a[1])
 	butterflyG2(&a[2], &a[3])