some bugs fixed in altivec.h

[Zeson Wu via cfe-commits]

It's mainly about signed and unsigned llvm builtins confusion.
When function args are bool vector and signed vector, it should be mapped
to signed version instruction.

reference: http://www.nxp.com/assets/documents/data/en/reference-
manuals/ALTIVECPIM.pdf

-- 
Zeson
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