APFloat: Add isSmallestNormalized predicate function

It was annoying to write the check for this in the one case I added,
and I'm planning on adding another, so add a convenient PatternMatch
like for other special case values.

I have no idea what is going on in the DoubleAPFloat case, I reversed
this from the makeSmallestNormalized test. Also could implement this
as *this == getSmallestNormalized() for less code, but this avoids the
construction of a temporary APFloat copy and follows the style of the
other functions.