Fix an issue in SimplifySetCC() specific to vector comparisons.

Fix an issue in SimplifySetCC() specific to vector comparisons.

When folding X == X we need to check getBooleanContents() to determine if the
result is a vector of ones or a vector of negative ones.

I tried creating a test case, but the problem seems to only be exposed on a
much older version of clang (around r144500).
rdar://10923049

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@153966 91177308-0d34-0410-b5e6-96231b3b80d8