[turbofan] Correctify lowering of Uint8ClampedArray buffer access.

TEST=mjsunit/compiler/regress-444508.js
BUG=chromium:444508
LOG=y
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/794013004

Cr-Commit-Position: refs/heads/master@{#25916}