In general, TurboFan doesn't encounter phi nodes with only a single
data input in the backend. However, CSA-based builtins (especially
auto-generated ones, e.g. from Torque), may contain single-input phi nodes,
although outside the auto-generated case this doesn't happen much in practice.

Single input phi nodes (i.e. phis in blocks with one predecessor) don't have
any side effects and are essentially useless and harmless, but to avoid problems
in the backend of TurboFan (whose SSA deconstruction disallows control flow
splits that continue to blocks with phis), this CL tweaks the existing
CSA-only control flow and graph sanitization in the CSA path to ensure
no no-op phis.

Change-Id: I109f4dc6cde5ad1794585a09609a230b1848e0d5
Reviewed-on: https://chromium-review.googlesource.com/963711Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Daniel Clifford <danno@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52056}