The problem occurs with negative numbers, as the 32-bit input values
are sign extended into the 64-bit registers. When doing:

  // x |= (x >> 1);
  // x |= (x >> 2);
  // x |= (x >> 4);
  // x |= (x >> 8);
  // x |= (x >> 16);
  // return (WORDBITS - popc(x));

    __ srl(Rsrc, 1, Rtmp);
    __ or3(Rsrc, Rtmp, Rdst);
    <snip>

the 32-bit logical right shift clears the upper 32 bits but the OR
with the source value adds in the sign extended upper 32 bits again.
This leads to a POPC value for e.g. -1 of 64 instead of 32.

The fix is to zero-extend the 32-bit source value before applying the
algorithm on it.

Additionally two typos are fixed.