[gdb/testsuite] Fix control-flow in gdb.reverse/insn-reverse.exp

In gdb.reverse/insn-reverse.exp, we have loop containing a call to
gdb_test_multiple, which itself contains a break:
...
  for {} {$count < 500} {incr count} {
    ...
    gdb_test_multiple "x/i \$pc" "" {
      ...
      break
    }
...

On SLE-11 with:
...
$ runtest --version
Expect version is       5.44.1.11
Tcl version is          8.5
Framework version is    1.4.4
...
the break doesn't seem to have the effect of breaking out of the loop.

The break does have the effect of terminating evaluation of the expect clause,
which means we don't set insn_array, after which we run into:
...
ERROR: tcl error sourcing /tmp/tdevries/src/gdb/testsuite/gdb.reverse/insn-reverse.exp.
ERROR: can't read "insn_array(5)": no such element in array
...

---
 gdb/testsuite/gdb.reverse/insn-reverse.exp | 26 +++++++++++++++-----------
 1 file changed, 15 insertions(+), 11 deletions(-)

diff --git a/gdb/testsuite/gdb.reverse/insn-reverse.exp b/gdb/testsuite/gdb.reverse/insn-reverse.exp
index 39e276964f9..d1fdebf75fe 100644
--- a/gdb/testsuite/gdb.reverse/insn-reverse.exp
+++ b/gdb/testsuite/gdb.reverse/insn-reverse.exp
@@ -79,23 +79,27 @@ proc test { func testcase_nr } {
 	set count 0
 	set insn_addr ""
 	for {} {$count < 500} {incr count} {
+	    set prev_insn_addr $insn_addr
+	    set insn_addr ""
 	    gdb_test_multiple "x/i \$pc" "" {
 		-re ".* ($hex) <.*>:\[ \t\]*(.*)\r\n$gdb_prompt $" {
-		    set prev_insn_addr $insn_addr
 		    set insn_addr $expect_out(1,string)
+		    set insn_array($count) $expect_out(2,string)
+		}
+	    }
 
-		    if [expr {$last_insn == $insn_addr}] {
-			break
-		    }
+	    if [expr {$insn_addr == ""}] {
+		break
+	    }
 
-		    if [expr {$prev_insn_addr == $insn_addr}] {
-			# Failed to make progress, might have run into SIGILL.
-			unsupported "no progress at: $expect_out(2,string)"
-			break
-		    }
+	    if [expr {$last_insn == $insn_addr}] {
+		break
+	    }
 
-		    set insn_array($count) $expect_out(2,string)
-		}
+	    if [expr {$prev_insn_addr == $insn_addr}] {
+		# Failed to make progress, might have run into SIGILL.
+		unsupported "no progress at: $expect_out(2,string)"
+		break
 	    }
 
 	    set pre_regs($count) [capture_command_output "info all-registers" ""]