math: Ldexp doesn't handle denormals correctly

[crvv]

What version of Go are you using (go version)?

1.9.2 and tip

Does this issue reproduce with the latest release?

Yes

What did you do?

https://play.golang.org/p/3OfmZgFcjRR

What did you expect to see?

math.Ldexp(1, -1075) = 0
math.Ldexp(1.000000000000001, -1075) = 5e-324
math.Ldexp(2, -1075) = 5e-324

What did you see instead?

math.Ldexp(1, -1075) = 0
math.Ldexp(1.000000000000001, -1075) = 0
math.Ldexp(2, -1075) = 5e-324

The result of ldexp_386.s is correct.
This issue is that the function ldexp written in Go doesn't handle denormals correctly.

[gopherbot]

Change https://golang.org/cl/87338 mentions this issue: math: fix Ldexp when result is below 5e-324

[odeke-em]

/cc @griesemer @ALTree @ericlagergren