[InstSimplify] add tests for rotate; NFC
[InstSimplify] add tests for rotate; NFC

Rotate is a special-case of funnel shift that has different
poison constraints than the general case. That's not visible
yet in the existing tests, but it needs to be corrected.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@354894 91177308-0d34-0410-b5e6-96231b3b80d8

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