[dev.ssa] cmd/compile: implement "if SETEQ" branches

Change-Id: I814fd0c2f1a622cca7dfd1b771f81de309a1904c
Reviewed-on: https://go-review.googlesource.com/12441Reviewed-by: Keith Randall <khr@golang.org>

src/cmd/compile/internal/ssa/gen/AMD64.rules

@@ -95,6 +95,7 @@
 
 // block rewrites
 (If (SETL cmp) yes no) -> (LT cmp yes no)
+(If (SETEQ cmp) yes no) -> (EQ cmp yes no)
 (If (SETNE cmp) yes no) -> (NE cmp yes no)
 (If (SETB cmp) yes no) -> (ULT cmp yes no)
 (If cond yes no) && cond.Op == OpAMD64MOVBload -> (NE (TESTB <TypeFlags> cond cond) yes no)

src/cmd/compile/internal/ssa/rewriteAMD64.go

@@ -2142,6 +2142,26 @@ func rewriteBlockAMD64(b *Block) bool {
 		}
 		goto ende4d36879bb8e1bd8facaa8c91ba99dcc
 	ende4d36879bb8e1bd8facaa8c91ba99dcc:
+		;
+		// match: (If (SETEQ cmp) yes no)
+		// cond:
+		// result: (EQ cmp yes no)
+		{
+			v := b.Control
+			if v.Op != OpAMD64SETEQ {
+				goto endf113deb06abc88613840e6282942921a
+			}
+			cmp := v.Args[0]
+			yes := b.Succs[0]
+			no := b.Succs[1]
+			b.Kind = BlockAMD64EQ
+			b.Control = cmp
+			b.Succs[0] = yes
+			b.Succs[1] = no
+			return true
+		}
+		goto endf113deb06abc88613840e6282942921a
+	endf113deb06abc88613840e6282942921a:
 		;
 		// match: (If (SETNE cmp) yes no)
 		// cond: