[LV][LAA] Vectorize math lib calls with mem write-only attribute

Teach LAA to consider safe specific math lib calls which are known to
have set the memory write-only attribute. Those attributes are set to
calls by inferNonMandatoryLibFuncAttrs, in BuildLibCalls.cpp, and the
current ones are modf/modff and frexp/frexpf.

This happens only when the calls are found through TLI to have
vectorized counterparts.