This is a reland of d504203e

Original change's description:
> [turbofan] Consistently use String feedback for JSAdd.
> 
> Currently we didn't always consistently use the String feedback on
> JSAdd, but only if JSTypedLowering would already figure out statically
> that one of the inputs is already a String. That leads to some odd
> performance cliffs, as highlighted in the referenced bug.
> 
> This CL fixes the JSTypedLowering::ReduceJSAdd to always bake in the
> String feedback. This improves the relevant performance tests from the
> bug from
> 
>   console.timeEnd: Runtime join3, 967.512000
>   console.timeEnd: Runtime join, 1004.599000
>   console.timeEnd: Runtime join3, 1124.764000
>   console.timeEnd: Runtime join, 966.164000
>   console.timeEnd: Runtime join3, 1145.296000
>   console.timeEnd: Runtime join, 966.176000
>   console.timeEnd: Runtime join3, 1145.272000
>   console.timeEnd: Runtime join, 931.266000
> 
> to
> 
>   console.timeEnd: Runtime join3, 903.050000
>   console.timeEnd: Runtime join, 856.509000
>   console.timeEnd: Runtime join3, 945.144000
>   console.timeEnd: Runtime join, 840.038000
>   console.timeEnd: Runtime join3, 927.965000
>   console.timeEnd: Runtime join, 841.263000
>   console.timeEnd: Runtime join3, 929.342000
>   console.timeEnd: Runtime join, 858.143000
> 
> which corresponds to an 8-18% improvement.
> 
> Bug: v8:7415
> Change-Id: I62e008298e4ee0864885b37817c91d055acf2a09
> Reviewed-on: https://chromium-review.googlesource.com/936643
> Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
> Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51551}

Bug: v8:7415
Change-Id: Idbc889dc070f764dacf3bf535e41d0e66ab09bea
Reviewed-on: https://chromium-review.googlesource.com/964421Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51965}