cmd/compile: remove outdated comment in prove.go

See the cases above all of theses are implemented (except XOR which has a blank case with comments).

Change-Id: I9e2994490dac89e86ba70c1abeb1af1cbcf032e9
Reviewed-on: https://go-review.googlesource.com/c/go/+/604416
Reviewed-by: Keith Randall <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: David Chase <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: Jorropo

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

@@ -1788,7 +1788,6 @@ func (ft *factsTable) flowLimit(v *Value) bool {
 		}
 		return ft.newLimit(v, l)
 	}
-	// TODO: mul/lsh, rsh, div/mod, and/or/xor
 	return false
 }