Comb Loop

Created by: davideschiavone

This functional fix:

https://github.com/pulp-platform/riscv/commit/f19f0722b06b53a59cb38ea982d909f2d8797d13

introduced a combinational loop in the prefetcher 32 bit version:

hwlp_masked --> fifo_i/in_valid_i --> fifo_i/out_valid_o --> hwlp_masked

Thanks @igorloi to find it!