cmd/compile: update comment about OpPhi argLength

We use -1 for Phi argLength, not "a really large number".

Change-Id: I5d4fa364c22c3cf40213f2368a9a20de7e989aac
Reviewed-on: https://go-review.googlesource.com/c/go/+/203883
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/ssa/gen/genericOps.go

@@ -316,8 +316,7 @@ var genericOps = []opData{
 	// See section 7.2 in ieee754.
 	{name: "Fma", argLength: 3}, // compute (a*b)+c without intermediate rounding
 
-	// Data movement, max argument length for Phi is indefinite so just pick
-	// a really large number
+	// Data movement. Max argument length for Phi is indefinite.
 	{name: "Phi", argLength: -1, zeroWidth: true}, // select an argument based on which predecessor block we came from
 	{name: "Copy", argLength: 1},                  // output = arg0
 	// Convert converts between pointers and integers.