[PowerPC] Fix vector equality comparison for v2i64 pre-Power8

The current code makes the assumption that equality
comparison can be performed with a word comparison
instruction. While this is true if the entire 64-bit
results are used, it does not generally work. It is
possible that the low order words and high order
words produce different results and a user of only
one will get the wrong result.

This patch adds an and of the result words so that
each word has the result of the comparison of the
entire doubleword that contains it.

Differential revision: https://reviews.llvm.org/D115678