[PATCH] [APFloat] Fix a bug in method 'fusedMultiplyAdd' (fixes PR20832).

[Andrea Di Biagio]

Hi,

When folding a fused multiply-add builtin call, make sure that we propagate the correct result in the case where the addend is zero, and the two other operands are finite non-zero.

Example,

```
define double @test() {
  %1 = call double @llvm.fma.f64(double 7.0, double 8.0, double 0.0)
  ret double %1
}
```

Before this patch, the instruction simplifier wrongly folded the builtin call in function @test to constant 'double 7.0'.
With this patch, method 'fusedMultiplyAdd' now correctly evaluates the multiply and returns the expected result (i.e. 56.0).

Added test fold-builtin-fma.ll with the reproducible from PR20832 plus extra test cases to verify the behavior of method 'fusedMultiplyAdd' in the presence of NaN/Inf operands.

Please let me know if ok to submit.

Thanks,
Andrea Di Biagio

http://reviews.llvm.org/D5152

Files:
  lib/Support/APFloat.cpp
  test/Transforms/InstSimplify/fold-builtin-fma.ll
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