[APFloat] Convert all references to fcNormal to references to isFiniteNonZero().

Currently inside APFloat fcNormal still implies the old definition of Normal
(i.e. isFiniteNonZero) instead of the proper IEEE-754R definition that the
external method isNormal() uses.

This patch prepares for the internal switch inside APFloat by converting all
references that check if a category is fcNormal directly with an indirect call
via isFiniteNonZero().
In InstCombine{AddSub,MulDivRem} convert APFloat.isFiniteNonZero() && !APFloat.isDenormal => APFloat.isNormal.