Ensure all uses of permute instructions feed vector stores

There is a problem in VSXSwapRemoval where it is incorrectly removing permute instructions.
In this case, the permute is feeding both a vector store and also a non-store instruction. In this case, the permute cannot be removed.

The fix is to simply look at all the uses of the vector register defined by the permute and ensure that all the uses are vector store instructions.

This problem was reported in PR 27735 (https://llvm.org/bugs/show_bug.cgi?id=27735).

Test case based on the original problem reported.

Phabricator Review: http://reviews.llvm.org/D21802

llvm-svn: 274645

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