Fix UBSan test error from r248897 about left shift of unsigned value.
Fix UBSan test error from r248897 about left shift of unsigned value.

Fixed by masking off the upper bits that we are shifting off before
doing the left shift.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@249689 91177308-0d34-0410-b5e6-96231b3b80d8

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