Explicitly check for presence of correct results in instcombine memmove test

Summary:
Rather than testing for expected results, test/Transforms/InstCombine/memmove.ll is testing for the absence of calls to llvm.memmove.

In the case of test3, the test has stopped testing for materialization of loads/stores, but remained passing due to testing for the absence of llvm.memset calls rather than the presence of load/store instructions. Right now this test ends up with an empty function body because the alloca is eliminated as safe-to-remove, which results in the llvm.memmove calls being eliminated due to a pointer arg being undef; so it is not testing for conversion of llvm.memmove into load/store instructions at all.

Reviewers: eli.friedman, anna, efriedma

Reviewed By: efriedma

Subscribers: efriedma, llvm-commits

Differential Revision: https://reviews.llvm.org/D34645

llvm-svn: 306679