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

llvm-svn: 153966