powerpc optimiser incorrectly failed to realise that,
almost uniquely, RLWMI does a read-modify-write of its destination,
and therefore:
1. peep.c must mark the instruction as "read, read, rar" in copyu
2. peep.c must not allow the instruction to act like a move in subprop (in the sense that it fully-replaces a register's contents).

the error affected only programs that used right shifts of vlongs.

Reference: /n/sources/patch/applied/qc-peep
Date: Mon Mar  7 01:11:50 CET 2011
Signed-off-by: forsyth@terzarima.net

--- /sys/src/cmd/qc/peep.c	Mon Mar  7 01:07:41 2011
+++ /sys/src/cmd/qc/peep.c	Mon Mar  7 01:07:38 2011
@@ -423,8 +423,6 @@
 		case AREMU:
 		case ARLWNM:
 		case ARLWNMCC:
-		case ARLWMI:
-		case ARLWMICC:
 
 		case AFADD:
 		case AFADDS:
@@ -621,7 +619,6 @@
 			print(" (???)");
 		return 2;
 
-
 	case ANOP:	/* read, write */
 	case AMOVW:
 	case AMOVH:
@@ -666,6 +663,12 @@
 			return 1;
 		return 0;
 
+	case ARLWMI:	/* read read rar */
+	case ARLWMICC:
+		if(copyas(&p->to, v))
+			return 2;
+		/* fall through */
+
 	case AADD:	/* read read write */
 	case AADDC:
 	case AADDE:
@@ -693,8 +696,6 @@
 	case ADIVWU:
 	case AREM:
 	case AREMU:
-	case ARLWMI:
-	case ARLWMICC:
 	case ARLWNM:
 	case ARLWNMCC: