cmd/compile: rewrite to elide Slicemask from len==c>0 slicing

This might have been something that prove could be educated
into figuring out, but this also works, and it also helps
prove downstream.

Adjusted the prove test, because this change moved a message.

Cherry-picked from the dev.simd branch. This CL is not
necessarily SIMD specific. Apply early to reduce risk.

Change-Id: I5eabe639eff5db9cd9766a6a8666fdb4973829cb
Reviewed-on: https://go-review.googlesource.com/c/go/+/697715
Commit-Queue: David Chase <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
TryBot-Bypass: David Chase <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/708858
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Junyang Shao <[email protected]>

Author: dr2chase

src/cmd/compile/internal/ssa/_gen/generic.rules

@@ -989,6 +989,10 @@
     (Const64 <typ.Int> [0])
     (Const64 <typ.Int> [0]))
 
+// Special rule to help constant slicing; len > 0 implies cap > 0 implies Slicemask is all 1
+(SliceMake (AddPtr <t> x (And64 y (Slicemask _))) w:(Const64 [c]) z) && c > 0 => (SliceMake (AddPtr <t> x y) w z)
+(SliceMake (AddPtr <t> x (And32 y (Slicemask _))) w:(Const32 [c]) z) && c > 0 => (SliceMake (AddPtr <t> x y) w z)
+
 // interface ops
 (ConstInterface) =>
   (IMake

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

@@ -511,10 +511,10 @@ func f19() (e int64, err error) {
 
 func sm1(b []int, x int) {
 	// Test constant argument to slicemask.
-	useSlice(b[2:8]) // ERROR "Proved slicemask not needed$"
+	useSlice(b[2:8]) // optimized away earlier by rewrite
 	// Test non-constant argument with known limits.
 	if cap(b) > 10 {
-		useSlice(b[2:])
+		useSlice(b[2:]) // ERROR "Proved slicemask not needed$"
 	}
 }