[LoopVectorizer] Require no-signed-zeros-fp-math=true for fmin/fmax

Currently, setting the `no-nans-fp-math` attribute to true will allow
loops with fmin/fmax to vectorize, though we should be requiring that
`no-signed-zeros-fp-math` is also set.

This patch adds the check for no-signed-zeros at the function level and includes
tests to make sure we don't vectorize functions with only one of the attributes
associated.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D96604