This addresses the remaining issue from D87188. Due to a series of folds, we may end up with abs-of-abs represented as `x == 0 ? -abs(x) : abs(x)`. Rather than recognizing this as a special abs pattern and doing an abs-of-abs fold on it afterwards, I'm directly folding this to one of the select operands in InstSimplify.

The general pattern falls into the "select with operand replaced" category, but that fold is not powerful enough to recognize that both hands of the select are the same for value zero.