freedreno: Use the NIR lowering for isign.

I think this will save an instruction and hopefully not increase any other
costs (possibly the immediate -1 and 1?), but I haven't actually tested.

Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
24 jobs for nir-isign in 152 minutes and 11 seconds (queued for 2 minutes and 2 seconds)
Status Job ID Name Coverage
  Build+Test
passed #115520
build:make-gallium-drivers-other

00:12:44

passed #115519
build:make-gallium-drivers-radeonsi

00:07:20

passed #115518
build:make-gallium-drivers-swr

00:23:14

passed #115521
build:make-gallium-st-clover-llvm-39

00:17:01

passed #115522
build:make-gallium-st-clover-llvm-4

00:15:48

passed #115523
build:make-gallium-st-clover-llvm-5

00:06:47

passed #115524
build:make-gallium-st-clover-llvm-6

00:06:52

passed #115525
build:make-gallium-st-clover-llvm-7

00:07:33

passed #115526
build:make-gallium-st-other

00:13:29

passed #115517
build:make-loader-classic-dri

00:08:44

passed #115516
build:make-vulkan

00:06:19

passed #115512
build:meson-gallium-clover-llvm5

00:04:49

passed #115513
build:meson-gallium-clover-llvm6

00:03:17

passed #115514
build:meson-gallium-clover-llvm7

00:04:58

passed #115511
build:meson-gallium-drivers-other

00:05:10

passed #115510
build:meson-gallium-radeonsi

00:04:15

passed #115515
build:meson-gallium-st-other

00:04:25

passed #115509
build:meson-gallium-swr

00:24:49

passed #115508
build:meson-glvnd

00:07:20

passed #115507
build:meson-loader-classic-dri

00:09:25

passed #115506
build:meson-vulkan

00:02:11

passed #115528
build:scons-llvm

00:03:53

passed #115527
build:scons-nollvm

00:03:40

passed #115529
build:scons-swr

00:05:46