[MLIR][Arith] Fold trunci with ext if the bit width of the input type of ext is greater than the

This patch is mainly to deal with folding trunci with ext，as flows：
    trunci(zexti(a)) -> trunci(a)
    trunci(zexti(a)) -> trunci(a)

Reviewed By: ftynse

Differential Revision: https://reviews.llvm.org/D140604