cmd/compile: x+0 != x if x is float -0.

[randall77]

We need to make sure we don't do the optimization x+0 -> x for floats, because it doesn't work when x is negative zero.

This bug started with 1.8.
CL: https://go-review.googlesource.com/c/go/+/24790

[gopherbot]

Change https://golang.org/cl/135701 mentions this issue: cmd/compile: don't do floating point optimization x+0 -> x

[gopherbot]

Change https://golang.org/cl/190937 mentions this issue: test/fixedbugs: add more test cases to issue #27718