go.tools/ssa/interp: implement unary negation of complex numbers.

Fixes golang/go#6291.

Also: call go/types.DefaultSizeof instead of hard-coding the
initial value of runtime.sizeof_C_MStats.

R=gri
CC=golang-dev
https://golang.org/cl/13648043

ssa/interp/interp.go

@@ -571,12 +571,9 @@ func Interpret(mainpkg *ssa.Package, mode Mode, filename string, args []string)
 			setGlobal(i, pkg, "envs", envs)
 
 		case "runtime":
-			// TODO(gri): expose go/types.sizeof so we can
-			// avoid this fragile magic number;
-			// unsafe.Sizeof(memStats) won't work since gc
-			// and go/types have different sizeof
-			// functions.
-			setGlobal(i, pkg, "sizeof_C_MStats", uintptr(3696))
+			// (Assumes no custom Sizeof used during SSA construction.)
+			sz := types.DefaultSizeof(pkg.Object.Scope().Lookup("MemStats").Type())
+			setGlobal(i, pkg, "sizeof_C_MStats", uintptr(sz))
 
 		case "os":
 			Args := []value{filename}

ssa/interp/ops.go

@@ -797,6 +797,10 @@ func unop(instr *ssa.UnOp, x value) value {
 			return -x
 		case float64:
 			return -x
+		case complex64:
+			return -x
+		case complex128:
+			return -x
 		}
 	case token.MUL:
 		return copyVal(*x.(*value)) // load