cmd/compile: ternary rewrite of rewrite should skip, not panic

The panic was unnecessary, if there's nothing to rewrite,
just do nothing.  Added a debug message for this to help
with testing; it seems (from accidentally perturbing the
test away from failure) to be somewhat rare, so likely
okay to mingle with the other debugging output.

Fixes #77582.

Change-Id: I676396f4bb530cb6b55dfe543ad489f84710900d
Reviewed-on: https://go-review.googlesource.com/c/go/+/749241
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

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

@@ -5,7 +5,6 @@
 package ssa
 
 import (
-	"fmt"
 	"internal/goarch"
 	"slices"
 )
@@ -175,7 +174,10 @@ func rewriteTern(f *Func) {
 		imm := computeTT(a0, vars0)
 		op := ternOpForLogical(a0.Op)
 		if op == a0.Op {
-			panic(fmt.Errorf("should have mapped away from input op, a0 is %s", a0.LongString()))
+			if f.pass.debug > 0 {
+				f.Warnl(a0.Pos, "Skipping rewrite for %s, op=%v", a0.LongString(), op)
+			}
+			return
 		}
 		if f.pass.debug > 0 {
 			f.Warnl(a0.Pos, "Rewriting %s into %v of 0b%b %v %v %v", a0.LongString(), op, imm,

test/simd.go

@@ -8,7 +8,10 @@
 
 package foo
 
-import "simd/archsimd"
+import (
+	"fmt"
+	"simd/archsimd"
+)
 
 func f1(x archsimd.Int8x16) {
 	return // ERROR "has features avx"
@@ -143,3 +146,14 @@ func ternTricky3(x, y, z archsimd.Int32x8) archsimd.Int32x8 {
 	// a is a common subexpression
 	return a.Or(w) // ERROR "has features avx[+]avx2[+]avx512"  // This does not rewrite, do we want it to?
 }
+
+func vpternlogdPanic() {
+	resultsMask := archsimd.Mask64x8{}
+
+	for { // ERROR "has features avx+avx2+avx512"
+		resultsMask = archsimd.Mask64x8FromBits(0).Or( // ERROR "has features avx+avx2+avx512"
+			archsimd.Float64x8{}.Less(
+				archsimd.BroadcastFloat64x8(0))).Or(resultsMask) // ERROR "Rewriting.*ternInt" "Skipping rewrite"
+		fmt.Print(resultsMask.And(resultsMask.And(archsimd.Mask64x8{})))
+	}
+}