[mlir][Linalg] Drop unit-trip loops of reductions only if other reduction loops exists.

Recent change enable dropping unit-trip loops of "reduction" iterator
type as well. This is fine as long as there is one other "reduction"
iterator in the operation. Without this the initialized value (value
of `out`) is not read which leads to a correctness issue.

Also fix a bug in the `fill` -> `tensor_reshape` folding. The `out`
operand of the `fill` needs to be reshaped to get the `out` operand of
the generated `fill` operation.

Differential Revision: https://reviews.llvm.org/D100145