nir: add conditional discard optimisation (v4)

nir: add conditional discard optimisation (v4)

This is ported from GLSL and converts

if (cond)
	discard;

into
discard_if(cond);

This removes a block, but also is needed by radv
to workaround a bug in the LLVM backend.

v2: handle if (a) discard_if(b) (nha)
cleanup and drop pointless loop (Matt)
make sure there are no dependent phis (Eric)
v3: make sure only one instruction in the then block.
v4: remove sneaky tabs, add cursor init (Eric)

Reviewed-by: Eric Anholt <eric@anholt.net>
Cc: "13.0" <mesa-stable@lists.freedesktop.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>