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

[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.