[dart2js] Dead code eliminator should remove dead phis

The bad codegen reported in #48383 was due to an unused phi confusing
the expression-tree construction algorithm. It was trying to generate

    data = cond ? callWithSideEffects() : null;

but `data` is unused, so somehow we generated only

    cond;

We should not have unused phis at this point so I put a safety check
in the above code to not try to build an unused conditional, and
changed the dead-code eliminator to eliminate dead phis.

Now we get

    if (cond)
      callWithSideEffects();

I compared the output for some large apps and there were about a dozen
changes. All looked like improvements, e.g. not assigning to an unused
variable. There was some code missing that is now present, but luckily
for the large apps, it was all code that did not have real-world side
effects.


Fixed: 48383
Change-Id: Id7b32cfa0cbfb47a4d9eff174ad9ae52da99f6a4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/232781
Reviewed-by: Sigmund Cherem <sigmund@google.com>
Commit-Queue: Stephen Adams <sra@google.com>