nir: add if opt opt_if_loop_last_continue()

nir: add if opt opt_if_loop_last_continue()

Removing the last continue can allow more loops to unroll. Also
inserting code into the if branch can allow the various if opts
to progress further.

The insertion of some loops into the if branch also reduces VGPR
use in some shaders.

vkpipeline-db results (VEGA):

Totals from affected shaders:
SGPRS: 6552 -> 6576 (0.37 %)
VGPRS: 6544 -> 6532 (-0.18 %)
Spilled SGPRs: 0 -> 0 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 481952 -> 478032 (-0.81 %) bytes
LDS: 13 -> 13 (0.00 %) blocks
Max Waves: 241 -> 242 (0.41 %)
Wait states: 0 -> 0 (0.00 %)

Shader-db results radeonsi (VEGA):

Totals from affected shaders:
SGPRS: 168 -> 168 (0.00 %)
VGPRS: 144 -> 140 (-2.78 %)
Spilled SGPRs: 157 -> 157 (0.00 %)
Spilled VGPRs: 0 -> 0 (0.00 %)
Private memory VGPRs: 0 -> 0 (0.00 %)
Scratch size: 0 -> 0 (0.00 %) dwords per thread
Code Size: 8524 -> 8488 (-0.42 %) bytes
LDS: 0 -> 0 (0.00 %) blocks
Max Waves: 7 -> 7 (0.00 %)
Wait states: 0 -> 0 (0.00 %)

v2: (Timothy Arceri):
- allow for continues in either branch
- move any trailing loops inside the if as well as blocks.
- leave nir_opt_trivial_continues() to actually remove the
  continue.

Reviewed-by: Thomas Helland <thomashelland90@gmail.com>
Signed-off-by: Timothy Arceri <tarceri@itsqueeze.com>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=32211