[mlir][tensor] Implement constant folder for tensor.pad (#92691)

Extend the folding ability of the RewriteAsConstant patterns to include
tensor.pad operations on constants. The new pattern with constant fold
tensor.pad operations which operate on tensor constants and have
statically resolvable padding sizes/values.

    %init = arith.constant dense<[[6, 7], [8, 9]]> : tensor<2x2xi32>
    %pad_value = arith.constant 0 : i32

    %0 = tensor.pad %init low[1, 1] high[1, 1] {
      ^bb0(%arg1: index, %arg2: index):
        tensor.yield %pad_value : i32
    } : tensor<2x2xi32> to tensor<4x4xi32>

becomes

    %cst = arith.constant dense<[[0, 0, 0, 0],
                                 [0, 6, 7, 0],
                                 [0, 8, 9, 0],
                                 [0, 0, 0, 0]]> : tensor<4x4xi32>

Co-authored-by: Spenser Bauman <sabauma@fastmail>