Improve performance of PowerPC64 fast_long_increment_function

Improve performance of PowerPC64 fast_long_increment_function

Detecting overflow with the XER is slow, partially because we have to
clear it before use.

gcc does a better job of detecting overflow of an increment or decrement
than we can with inline assembly. It knows that an increment will only
overflow if it is one less than the overflow value. This means we end
up with a simple compare/branch. Furthermore, leaving it in c allows gcc
to schedule the instructions better.

This is 6% faster on a POWER8 running a simple testcase:

<?php
        function testcase($count = 100000000) {
		$x = 1;
                for ($i = 0; $i < $count; $i++) {
                        $x++;
                        $x++;
                        $x++;
                        $x++;
                        $x++;
                }
        }

        testcase();
?>