[Analysis][LoopVectorize] rename "Unsafe" variables/methods; NFC

Similar to b3a33553, but this shows a TODO and a potential
miscompile is already present.

We are tracking an FP instruction that does *not* have FMF (reassoc)
properties, so calling that "Unsafe" seems opposite of the common
reading.

I also removed one getter method by rolling the null check into
the access. Further simplification may be possible.

The motivation is to clean up the interactions between FMF and
function-level attributes in these classes and their callers.

The new test shows that there is an existing bug somewhere in
the callers. We assumed that the original code was fully 'fast'
and so we produced IR with 'fast' even though it was just 'reassoc'.