[MVE] Fold fadd(select(..., +0.0)) into a predicated fadd

We already have patterns for matching fadd(select(..., -0.0)),
but an upcoming patch will lead to patterns using +0.0 as the
identity instead of -0.0. I'm adding support for these patterns
now to avoid any regressions for MVE.

Differential Revision: https://reviews.llvm.org/D127275