[turbofan] Remove int32 narrowing during typed lowering.

[turbofan] Remove int32 narrowing during typed lowering.

With Int32Add we lose the int/uint distinction, so later, in simplified lowering we can make a wrong decision. E.g., see the attached test case, where we lower NumberAdd -> Int32Add because inputs are Uint32, but during simplified lowering we change the inputs to Int32, so we get a wrong result.

Simplified lowering will lower the NumberAdd operations anyway, so we should lose performance.

BUG=
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/721723004

Cr-Commit-Position: refs/heads/master@{#25368}