Previously we had to run SimplifiedLowering, ChangeLowering and
JSGenericLowering independently and exactly in this order to
achieve great performance for the common case of JSToNumber (i.e.
input is already a Smi or a HeapNumber). This phase ordering
problem already causes trouble with not being able to run the
generic lowering phase earlier, but also blocks proper plain
primitive ToNumber optimizations. So this properly integrates
JSToNumber into the truncation analysis and optimizes according
to the truncation (either Word32 or Float64).

R=jarin@chromium.org

Review URL: https://codereview.chromium.org/1901803002

Cr-Commit-Position: refs/heads/master@{#35643}