The code-generator used i.InputInt6 to get the mask-width from the
instruction. However, thereby 64 got wrapped to 0, which is an invalid
mask width. I changed the i.InputInt6 to an i.InputInt32, which should
be okay because the mask-width comes from base::bits::CountPopulation64.

BUG=v8:6122
R=bmeurer@chromium.org, v8-arm-ports@googlegroups.com

Review-Url: https://codereview.chromium.org/2755373002
Cr-Commit-Position: refs/heads/master@{#43927}