nir/loop_unroll: Prepare loop for unrolling in wrapper_unroll

Without loop_prepare_for_unroll loops are losing phis.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=111411
Fixes: 5db98195 "nir: add loop unroll support for wrapper loops"
Signed-off-by: Danylo Piliaiev <danylo.piliaiev@globallogic.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
22 jobs for !1710 with fix/while-false-loops-unroll in 16 minutes and 51 seconds (queued for 2 seconds)
latest detached
Status Name Job ID Coverage
  Containers Build
passed debian #521364

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  Build+Test
passed meson-arm64 #521373

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passed meson-armhf #521372

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passed meson-clang #521366

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passed meson-clover #521370

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passed meson-i386 #521374

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passed meson-main #521369

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passed meson-swr-glvnd #521365

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passed meson-vulkan #521371

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passed scons-llvm #521376

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passed scons-nollvm #521375

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passed scons-swr #521367

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passed scons-win64 #521368

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  Test
passed test-llvmpipe-gles2 1/4 #521377

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passed test-llvmpipe-gles2 2/4 #521378

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passed test-llvmpipe-gles2 3/4 #521379

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passed test-llvmpipe-gles2 4/4 #521380

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passed test-softpipe-gles2 1/4 #521381

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passed test-softpipe-gles2 2/4 #521382

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passed test-softpipe-gles2 3/4 #521383

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passed test-softpipe-gles2 4/4 #521384

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passed test-softpipe-gles3-limited #521385

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