cmd/compile/internal/gc: avoid potential endless loop in float printing

The compiler should not usually call Fconv with an infinity, but if
it does, Fconv will end in an endless loop. Test for infinities early.

Change-Id: I48f366466538b0bd26a851e01258725025babaff
Reviewed-on: https://go-review.googlesource.com/16777


Reviewed-by: Russ Cox <rsc@golang.org>