The conversion from -0.0 to +0.0 happens inside mpgetflt now.
The SSA code doesn't need this fix any more.

Change-Id: I6cd4f4a4e75b13cf284ebbb95b08af050ed9891c
Reviewed-on: https://go-review.googlesource.com/18942Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Keith Randall <khr@golang.org>