[APInt] Cast more calls to add/sub/mul overflow functions to void. I missed the unittests in r300758.
[APInt] Cast more calls to add/sub/mul overflow functions to void. I missed the unittests in r300758.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@300773 91177308-0d34-0410-b5e6-96231b3b80d8

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