[turbofan] Fix missing ChangeUint32ToUint64 in lowering of LoadBuffer.

TEST=mjsunit/compiler/regress-444695
BUG=chromium:444695
LOG=y
R=hpayer@chromium.org

Review URL: https://codereview.chromium.org/824843002

Cr-Commit-Position: refs/heads/master@{#25932}