go/types: match cmd/compile's alignment for complex64

Fixes #17584.

Change-Id: I3af31cc1f2e9c906f3b73e77f3b092624ba78fbe
Reviewed-on: https://go-review.googlesource.com/31939
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/cmd/vet/testdata/asm.go

@@ -42,3 +42,4 @@ func noptr(x int)
 func wrapper(x int)
 
 func f15271() (x uint32)
+func f17584(x float32, y complex64)

src/cmd/vet/testdata/asm1.s

@@ -306,3 +306,10 @@ TEXT ·f15271(SB), NOSPLIT, $0-4
     // Return them.
     PEXTRD $0, X0, x+0(FP)
     RET
+
+// issue 17584
+TEXT ·f17584(SB), NOSPLIT, $12
+	MOVSS	x+0(FP), X0
+	MOVSS	y_real+4(FP), X0
+	MOVSS	y_imag+8(FP), X0
+	RET

src/go/types/sizes.go

@@ -79,6 +79,10 @@ func (s *StdSizes) Alignof(T Type) int64 {
 	if a < 1 {
 		return 1
 	}
+	// complex{64,128} are aligned like [2]float{32,64}.
+	if isComplex(T) {
+		a /= 2
+	}
 	if a > s.MaxAlign {
 		return s.MaxAlign
 	}