[turbofan] Fix register allocator verifier to allow 1-input phis.
[turbofan] Fix register allocator verifier to allow 1-input phis.

R=dcarney@chromium.org
BUG=

Review URL: https://codereview.chromium.org/860363002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#26191}

UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh,UniversalFuture/moosh