glsl: Remove unused ir_loop_jump::loop pointer.

glsl: Remove unused ir_loop_jump::loop pointer.

Commit 0c005bd7 intended to make ir_loop_jump::mode public, but also
accidentally added a new pointer to the enclosing loop.  Furthermore, it
tried to initialize the new field by adding "this->loop = loop;" to the
constructor, but since there is no loop parameter, this only initialized
the field to itself---so it will likely be a garbage pointer.

A lot of code, such as lower_jumps, allocates new loop jumps without
setting this field appropriately, so any uses would probably just crash.

Thankfully, there were none, so we can just delete the field.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=51574
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Eric Anholt <eric@anholt.net>