[InstCombine] fold add+negate through select into sub

This transform came up as a potential DAGCombine in D133282,
so I wanted to see how it escaped in IR too.

We do general folds in InstCombiner::SimplifySelectsFeedingBinaryOp()
by checking if either arm of a select simplifies when the trailing
binop is threaded into the select.

So as long as one side simplifies, it's a good fold to combine a
negate and add into 1 subtract.

This is an example with a zero arm in the select:
https://alive2.llvm.org/ce/z/Hgu_Tj

And this models the tests with a cancelling 'not' op:
https://alive2.llvm.org/ce/z/BuzVV_

Differential Revision: https://reviews.llvm.org/D133369