nir: remove restrictions on opt_if_loop_last_continue()

When I implemented opt_if_loop_last_continue() I had restricted
this pass from moving other if-statements inside the branch opposite
the continue. At the time it was causing extra regisiter pressure
in some shaders, however that no longer seems to be an issue.

Samuel Pitoiset noticed that making this pass more aggressive
significantly improved the performance of Doom on RADV. Below are
the statistics he gathered.

28717 shaders in 14931 tests
Totals:
SGPRS: 1267317 -> 1267549 (0.02 %)
VGPRS: 896876 -> 895920 (-0.11 %)
Spilled SGPRs: 24701 -> 26367 (6.74 %)
Code Size: 48379452 -> 48507880 (0.27 %) bytes
Max Waves: 241159 -> 241190 (0.01 %)

Totals from affected shaders:
SGPRS: 23584 -> 23816 (0.98 %)
VGPRS: 25908 -> 24952 (-3.69 %)
Spilled SGPRs: 503 -> 2169 (331.21 %)
Code Size: 2471392 -> 2599820 (5.20 %) bytes
Max Waves: 586 -> 617 (5.29 %)

The codesize increases is related to Wolfenstein II.

This gives +10% FPS with Doom on my Vega56.

Rhys Perry also benchmarked Doom on his VEGA64:

Before: 72.53 FPS
After:  80.77 FPS

shader-db results i965 (SKL):

total instructions in shared programs: 15029076 -> 15029877 (<.01%)
instructions in affected programs: 493251 -> 494052 (0.16%)
helped: 3
HURT: 374

total cycles in shared programs: 263387688 -> 263401720 (<.01%)
cycles in affected programs: 30658226 -> 30672258 (0.05%)
helped: 3
HURT: 374

total spills in shared programs: 9691 -> 9748 (0.59%)
spills in affected programs: 88 -> 145 (64.77%)
helped: 0
HURT: 4

total fills in shared programs: 22076 -> 22133 (0.26%)
fills in affected programs: 128 -> 185 (44.53%)
helped: 0
HURT: 4

LOST:   0
GAINED: 2

Both the gain and spill hurt are in Doom shaders which is similiar
to what we see on radeonsi ironically the Doom Vulkan shaders are
the most helped by this change.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
15 jobs for !502 with continue_opt_and_gcm in 11 minutes and 34 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Containers Build
passed #221239
ubuntu

00:00:22

 
  Build+Test
passed #221240
autotools

00:11:09

passed #221242
meson-clang

00:07:15

passed #221248
meson-gallium-clover-llvm

00:04:49

passed #221249
meson-gallium-clover-llvm39

00:04:44

passed #221247
meson-gallium-drivers-other

00:06:32

passed #221246
meson-gallium-radeonsi

00:04:34

passed #221250
meson-gallium-st-other

00:02:49

passed #221241
meson-gallium-swr

00:09:19

passed #221245
meson-glvnd

00:04:27

passed #221244
meson-loader-classic-dri

00:05:20

passed #221243
meson-vulkan

00:03:15

passed #221252
scons-llvm

00:04:00

passed #221251
scons-nollvm

00:04:16

passed #221253
scons-swr

00:05:58