cmd/compile: don't generate algs for [0]T and [1]T

All [0]T values are equal.
[1]T values are equal iff their sole components are.

This types show up most frequently as a by-product of variadic
function calls, such as fmt.Printf("abc") or fmt.Printf("%v", x).

Cuts 12k off cmd/go and 22k off golang.org/x/tools/cmd/godoc, approx 0.1% each.

For #6853 and #9930

Change-Id: Ic9b7aeb8cc945804246340f6f5e67bbf6008773e
Reviewed-on: https://go-review.googlesource.com/19766Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

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

@@ -465,6 +465,15 @@ func algtype1(t *Type, bad **Type) int {
 			return a
 		}
 
+		switch t.Bound {
+		case 0:
+			// We checked above that the element type is comparable.
+			return AMEM
+		case 1:
+			// Single-element array is same as its lone element.
+			return a
+		}
+
 		return -1 // needs special compare
 
 	case TSTRUCT:

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

@@ -3193,6 +3193,21 @@ func walkcompare(np **Node, init **NodeList) {
 		return
 	}
 
+	if t.Etype == TARRAY {
+		// Zero- or single-element array, of any type.
+		switch t.Bound {
+		case 0:
+			finishcompare(np, n, Nodbool(n.Op == OEQ), init)
+			return
+		case 1:
+			l0 := Nod(OINDEX, l, Nodintconst(0))
+			r0 := Nod(OINDEX, r, Nodintconst(0))
+			a := Nod(n.Op, l0, r0)
+			finishcompare(np, n, a, init)
+			return
+		}
+	}
+
 	if t.Etype == TSTRUCT && countfield(t) <= 4 {
 		// Struct of four or fewer fields.
 		// Inline comparisons.