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[DAG] fold FP binops with undef operands to NaN

Description

[DAG] fold FP binops with undef operands to NaN

This is the FP sibling of D43141 with the corresponding IR change in rL327212.

We can't propagate undef here because if a variable operand is a NaN, these
binops must propagate NaN. Neither global nor node-level fast-math makes a
difference. If we have 'nnan', I think later folds can turn the NaN into undef.

The tests in X86/fp-undef.ll are meant to be the definitive verification for
these folds - everything reduces identically now.

The other test changes are collateral damage. They may need to be altered to
preserve their intent.

Differential Revision: https://reviews.llvm.org/D47026

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