[AArch64][SVE] Fold fadda(ptrue, x, select(mask, y, -0.0)) into fadda(mask, x, y)

This patch adds an SVE pattern to recognize the use of a select with an
fadda in the form fadda(ptrue, x, select(mask, y, -0.0)). In this case
the select can be folded away, with the select mask used as the
predicate for fadda. This improves the codegen when vectorizing loops
with ordered fp reductions.

Differential Revision: https://reviews.llvm.org/D129623