[mlir][tosa] Fix constant folding of tosa.mul

The constant folder for tosa.mul produces a tensor attribute whose type
may not match the result type of the operation when broadcasting is
needed. This results in a tosa.const op whose attribute's type does not
match the type of the const op.
This change explicitly expands the attribute to the expected result
type.

Reviewed By: eric-k256, jpienaar

Differential Revision: https://reviews.llvm.org/D150439