[DAG] Fold select(cond,binop(x,y),binop(x,z)) -> binop(x,select(cond,y,z))

Similar to the folds performed in InstCombinerImpl::foldSelectOpOp, this attempts to push a select further up to help merge a pair of binops.

I'm primarily interested in select(cond,add(x,y),add(x,z)) folds to help expose pointer math (see https://bugs.llvm.org/show_bug.cgi?id=51069 etc.) but I've tried to use the more generic isBinOp().

Differential Revision: https://reviews.llvm.org/D106058