aco: add fallback algorithm in get_reg()

The generated code is often terrible, but the situations where this is
needed are rare.

Signed-off-by: Rhys Perry <pendingchaos02@gmail.com>
36 jobs for !8713 with aco_ra_fallback in 14 seconds (queued for 2 seconds)
detached
Status Job ID Name Coverage
  Sanity
passed #6895670
sanity

00:00:14

 
  Container
manual #6895673
aarch64 manual
arm_build
manual #6895674
manual
arm_test-base
manual #6895675
windows shell 1809 mesa manual
windows_build_vs2019
manual #6895671
manual
x86_build-base
manual #6895672
manual
x86_test-base
 
  Container 2
created #6895683
android_build
created #6895686
arm64_test
created #6895687
armhf_test
created #6895680
i386_build
created #6895676
kernel+rootfs_amd64
created #6895677
aarch64
kernel+rootfs_arm64
created #6895678
aarch64
kernel+rootfs_armhf
created #6895681
ppc64el_build
created #6895682
s390x_build
created #6895679
x86_build
created #6895684
x86_test-gl
created #6895685
x86_test-vk
 
  Meson X86 64
created #6895693
meson-clang
created #6895694
meson-clover
created #6895690
meson-clover-testing
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meson-gallium
created #6895692
meson-release
created #6895688
meson-testing
created #6895689
meson-testing-asan
created #6895695
meson-vulkan
 
  Build Misc
created #6895696
meson-android
created #6895698
aarch64
meson-arm64
created #6895699
aarch64
meson-arm64-build-test
created #6895697
aarch64
meson-armhf
created #6895701
meson-i386
created #6895704
meson-mingw32-x86_64
created #6895703
kvm
meson-ppc64el
created #6895702
kvm
meson-s390x
created #6895700
allowed to fail
scons-win64
 
  Amd
created #6895705
radv-fossils