cmd/compile: handle min/max correctly in mayCall

CL 500575 changed mayCall to return "false" for min/max builtin.

However, with string or float, min/max requires runtime call, so mayCall
should return true instead. This's probably not a big problem, because
CL 506115 makes order pass handle min/max correctly. But it's still
better to do it the right way.

Updates golang#60582

Change-Id: I9779ca62bebd0f95e52ad5fa55b9160dc35b33aa
Reviewed-on: https://go-review.googlesource.com/c/go/+/506855
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/compile/internal/walk/walk.go

@@ -337,10 +337,14 @@ func mayCall(n ir.Node) bool {
 			n := n.(*ir.ConvExpr)
 			return ssagen.Arch.SoftFloat && (isSoftFloat(n.Type()) || isSoftFloat(n.X.Type()))
 
+		case ir.OMIN, ir.OMAX:
+			// string or float requires runtime call, see (*ssagen.state).minmax method.
+			return n.Type().IsString() || n.Type().IsFloat()
+
 		case ir.OLITERAL, ir.ONIL, ir.ONAME, ir.OLINKSYMOFFSET, ir.OMETHEXPR,
 			ir.OAND, ir.OANDNOT, ir.OLSH, ir.OOR, ir.ORSH, ir.OXOR, ir.OCOMPLEX, ir.OEFACE,
 			ir.OADDR, ir.OBITNOT, ir.ONOT, ir.OPLUS,
-			ir.OCAP, ir.OIMAG, ir.OLEN, ir.OREAL, ir.OMIN, ir.OMAX,
+			ir.OCAP, ir.OIMAG, ir.OLEN, ir.OREAL,
 			ir.OCONVNOP, ir.ODOT,
 			ir.OCFUNC, ir.OIDATA, ir.OITAB, ir.OSPTR,
 			ir.OBYTES2STRTMP, ir.OGETG, ir.OGETCALLERPC, ir.OGETCALLERSP, ir.OSLICEHEADER, ir.OSTRINGHEADER: