cmd/compile: fix cmpstring rewrite rule

We need to ensure that the Select0 lives in the same block as
its argument. Divide up the rule into 2 so that we can put the
parts in the right places.

(This would be simpler if we could use @block syntax mid-rule, but
that feature currently only works at the top level.)

This fixes the ssacheck builder after CL 578835

Change-Id: Id26a01d9fac0684e0b732d35d0f7999f6de07825
Reviewed-on: https://go-review.googlesource.com/c/go/+/580815
Auto-Submit: Keith Randall <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: randall77

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

@@ -2799,7 +2799,11 @@
 // same memory state can reuse the results of the first call.
 // See issue 61725.
 // Note that this could pretty easily generalize to any pure function.
-(StaticLECall {f} x y m:(SelectN [1] c:(StaticLECall {g} x y mem)))
+(SelectN [0] (StaticLECall {f} x y (SelectN [1] c:(StaticLECall {g} x y mem))))
   && isSameCall(f, "runtime.cmpstring")
   && isSameCall(g, "runtime.cmpstring")
-=> (MakeResult (SelectN [0] <typ.Int> c) m)
+=> @c.Block (SelectN [0] <typ.Int> c)
+
+// If we don't use the result of cmpstring, might as well not call it.
+// Note that this could pretty easily generalize to any pure function.
+(SelectN [1] c:(StaticLECall {f} _ _ mem)) && c.Uses == 1 && isSameCall(f, "runtime.cmpstring") && clobber(c) => mem