[PhaseOrdering] add explanatory comment for test; NFC

This note was going to be added in D129650, but we
solved the problem by reverting another patch, and
I missed transferring the comment.

diff --git a/llvm/test/Transforms/PhaseOrdering/SystemZ/sub-xor.ll b/llvm/test/Transforms/PhaseOrdering/SystemZ/sub-xor.ll
index 66870d1..23e45bc 100644 (file)
--- a/llvm/test/Transforms/PhaseOrdering/SystemZ/sub-xor.ll
+++ b/llvm/test/Transforms/PhaseOrdering/SystemZ/sub-xor.ll
@@ -6,6 +6,11 @@ target triple = "systemz"
 
 @ARR = internal global [100 x i32] zeroinitializer, align 4
 
+; This test uses 'sub' instructions for gep offsets to allow
+; codegen (LSR) to create optimal asm. If 'sub' is canonicalized
+; to 'xor', then the backend needs to be able to see through
+; that transform to produce optimal asm.
+
 define dso_local zeroext i32 @foo(ptr noundef %a) #0 {
 ; CHECK-LABEL: @foo(
 ; CHECK-NEXT:  entry: