When we eliminate nodes during truncation analysis that have no value
uses, we must make sure that we do not eliminate speculative number
operations that would have side effects depending on the inputs, i.e.
for example a SpeculativeNumberMultiply(x,y) does ToNumber(x) and
ToNumber(y) first, so if either x or y could throw an exception during
ToNumber conversion, we must not eliminate the multiplication, even if
it has no value uses (some later pass may kill the actual machine
multiplication, but the checks on the inputs have to remain still).
So we check whether both x and y are PlainPrimitive, i.e. neither
Receiver nor Symbol, which could raise exceptions for ToNumber, and
only in that case we propagate the "unusedness" of the node to its
inputs.

This also uncovered a bug with the type of Dead, which must be None,
as this represents an impossible value, so we had to fix that too.

Also the dead code removal will not work correctly for constants (i.e.
pure nodes with no value inputs), as those might be cached and hence
we might resurrect them for an unrelated node lowering during
SimplifiedLowering and only later kill the actual node (replacing its
uses with Dead), which would then also replace the new use with Dead.
So that was fixed as well. This shouldn't change anything for the
result, as unused constants automagically disappear from the graph later
on anyways.

R=yangguo@chromium.org
BUG=chromium:631318

Review-Url: https://codereview.chromium.org/2182003002
Cr-Commit-Position: refs/heads/master@{#38038}