[InstCombine] Use +0.0 instead of -0.0 as the FP identity for some folds

In foldSelectIntoOp we sometimes transform a select of a fadd into a
fadd of a select, where we select between data and an identity value.
For both fadd and fsub the identity is always -0.0, but if the nsz
flag is set on the select instruction we can use +0.0 instead. Doing
so then triggers other optimisations, such as when folding the select
of masked load into a new masked load.

Differential Revision: https://reviews.llvm.org/D126774