cmd/compile: diagnose constant division by complex zero

When casting an ideal to complex{64,128}, for example during the
evaluation of

  var a = complex64(0) / 1e-50

we want the compiler to report a division-by-zero error if a divisor
would be zero after the cast.

We already do this for floats; for example

  var b = float32(0) / 1e-50

generates a 'division by zero' error at compile time (because
float32(1e-50) is zero, and the cast is done before performing the
division).

There's no such check in the path for complex{64,128} expressions, and
no cast is performed before the division in the evaluation of

  var a = complex64(0) / 1e-50

which compiles just fine.

This patch changes the convlit1 function so that complex ideals
components (real and imag) are correctly truncated to float{32,64}
when doing an ideal -> complex{64, 128} cast.

Fixes #11674

Change-Id: Ic5f8ee3c8cfe4c3bb0621481792c96511723d151
Reviewed-on: https://go-review.googlesource.com/37891
Run-TryBot: Alberto Donizetti <alb.donizetti@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/compile/internal/gc/mpfloat.go

@@ -37,6 +37,13 @@ func newMpflt() *Mpflt {
 	return &a
 }
 
+func newMpcmplx() *Mpcplx {
+	var a Mpcplx
+	a.Real = *newMpflt()
+	a.Imag = *newMpflt()
+	return &a
+}
+
 func (a *Mpflt) SetInt(b *Mpint) {
 	if b.checkOverflow(0) {
 		// sign doesn't really matter but copy anyway