glsl: Stop tree grafting if a variable is overwritten as an 'out' param.

glsl: Stop tree grafting if a variable is overwritten as an 'out' param.

While reviewing some compiler cleanups I'd sent out, Paul noticed that
tree grafting wasn't taking "out" parameters into account.

Further investigation revealed that it isn't strictly necessary: ir_call
ends basic blocks, and tree grafting currently only operates on basic
blocks.  So calls already kill grafts.

However, just to be safe, this patch makes "out" parameters explicitly
kill grafts.  Paul and I both prefer this.  It's a bit clearer.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Paul Berry <stereotype441@gmail.com>