cmd/compile: never report "truncated to real" for toint calls
Whoever called toint() is expecting the {Mpint, Mpflt, Mpcplx} arg to
be converted to an integer expression, so it never makes sense to
report an error as "constant X truncated to real".
Fixes #11580
Change-Id: Iadcb105f0802358a7f77188c2b1e63fe80c5580c
Reviewed-on: https://go-review.googlesource.com/34638
TryBot-Result: Gobot Gobot <gobot@golang.org> Reviewed-by: Robert Griesemer <gri@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>