[dev.ssa] cmd/compile/internal/ssa: fix string slice types.

Change-Id: I28bc6373bb42d9abf4f179664dbaab8d514a6ab9
Reviewed-on: https://go-review.googlesource.com/14376
Reviewed-by: Keith Randall <khr@golang.org>
diff --git a/src/cmd/compile/internal/gc/ssa.go b/src/cmd/compile/internal/gc/ssa.go
index 70990bb..a554a1d 100644
--- a/src/cmd/compile/internal/gc/ssa.go
+++ b/src/cmd/compile/internal/gc/ssa.go
@@ -1722,12 +1722,12 @@
 		if n.Right.Left == nil {
 			low = zero
 		} else {
-			low = s.expr(n.Right.Left)
+			low = s.extendIndex(s.expr(n.Right.Left))
 		}
 		if n.Right.Right == nil {
 			high = len
 		} else {
-			high = s.expr(n.Right.Right)
+			high = s.extendIndex(s.expr(n.Right.Right))
 		}
 
 		// Panic if slice indices are not in bounds.