[turbofan] Weakening of types must weaken ranges inside unions.

BUG=
R=rossberg@chromium.org

Review URL: https://codereview.chromium.org/712623002

Cr-Commit-Position: refs/heads/master@{#25311}
Revert "[turbofan] Weakening of types must weaken ranges inside unions."

This reverts commit 4c1f4b796d1c455fc6a023abe145a5e48c4b7b1f.

TBR=rossberg@chromium.org

Review URL: https://codereview.chromium.org/722943003

Cr-Commit-Position: refs/heads/master@{#25312}