[gdb/testsuite] Fix count usage in gdb.reverse/insn-reverse.exp

Consider the test-case gdb.reverse/insn-reverse.exp.

After the loop setting count, the valid entries in various arrays range from 0
to $count - 1 inclusive.

Then $count is decremented:
...
       incr count -1
...
after which the valid entries range from 0 to $count inclusive.

The first subsequent loop handles that properly:
...
       for {set i $count} {$i >= 0} {incr i -1} {
...
but the following loop does not, because it treats $count as exclusive bound:
...
	for {set i 0} {$i < $count} {incr i} {
...

Fix this by removing the incr, and using $count - 1 as starting value in the
first loop.

gdb/testsuite/ChangeLog:

2020-12-04  Tom de Vries  <tdevries@suse.de>

	* gdb.reverse/insn-reverse.exp: Fix count handling.

2 files changed, 5 insertions, 2 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index e7408a0..a88668d 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,9 @@
 2020-12-04  Tom de Vries  <tdevries@suse.de>
 
+	* gdb.reverse/insn-reverse.exp: Fix count handling.
+
+2020-12-04  Tom de Vries  <tdevries@suse.de>
+
 	* gdb.reverse/insn-reverse-x86.c: Guard x86_64 assembly with #ifdef
 	__x86_64__.
 
diff --git a/gdb/testsuite/gdb.reverse/insn-reverse.exp b/gdb/testsuite/gdb.reverse/insn-reverse.exp
index d60f873..5831952 100644
--- a/gdb/testsuite/gdb.reverse/insn-reverse.exp
+++ b/gdb/testsuite/gdb.reverse/insn-reverse.exp
@@ -102,9 +102,8 @@ proc test { func testcase_nr } {
 	    gdb_test "si" "" ""
 	}
 
-	incr count -1
 	# Registers contents after each backward single step.
-	for {set i $count} {$i >= 0} {incr i -1} {
+	for {set i [expr $count - 1]} {$i >= 0} {incr i -1} {
 	    gdb_test "reverse-stepi" "" ""
 	    set post_regs($i) [capture_command_output "info all-registers" ""]
 	}