[mlir] Canonicalize dynamic tensor.pad ops with constant inputs

This commit adds a canonicalization pattern for tensor.pad which changes the output type to static at each dimension where the input shape is static and the high and low operands are constants. This corrects an issue arising in Torch-MLIR where pad ops would sometimes introduce dynamic shapes unnecessarily.

Reviewed By: raikonenfnu

Differential Revision: https://reviews.llvm.org/D143135