add a case Nate sent me

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24195 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/PowerPC/README.txt

@@ -217,3 +217,26 @@ _test:
         stw r2, 0(r3)
         blr
 
+===-------------------------------------------------------------------------===
+
+Compile this:
+
+int %f1(int %a, int %b) {
+        %tmp.1 = and int %a, 15         ; <int> [#uses=1]
+        %tmp.3 = and int %b, 240                ; <int> [#uses=1]
+        %tmp.4 = or int %tmp.3, %tmp.1          ; <int> [#uses=1]
+        ret int %tmp.4
+}
+
+without a copy.  We make this currently:
+
+_f1:
+        rlwinm r2, r4, 0, 24, 27
+        rlwimi r2, r3, 0, 28, 31
+        or r3, r2, r2
+        blr
+
+The two-addr pass or RA needs to learn when it is profitable to commute an
+instruction to avoid a copy AFTER the 2-addr instruction.  The 2-addr pass
+currently only commutes to avoid inserting a copy BEFORE the two addr instr.
+