If we have good lower bound type information in simplified lowering
that the input to PlainPrimitiveToNumber is a string, then we'd like
to introduce a call to the StringToNumber builtin. However, this
requires more careful management of the effect chain than we had
previously. To fix this, introduce a StringToNumber opcode which
defers the graph alteration until effect-control-linearization,
when the effect chain is available for careful wiring.

Bug: v8:6929
Change-Id: I4f0e43fe474a44d0dfa095a3a01caece649d82db
Reviewed-on: https://chromium-review.googlesource.com/727934Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48741}