v3d: writes to magic registers aren't RF writes after THREND

Shaders must not attempt to write to the register files in the last
three instructions, but that doesn't include the magic registers:

nop                  ; nop               ; thrsw; ldtmu.- *** ERROR ***
nop                  ; nop
nop                  ; nop

v2: Simplify validation rules. (Eric Anholt)
v3: Adjust validation even more. (Eric Anholt)

Reviewed-by: Eric Anholt <eric@anholt.net>
22 jobs for !1722 with ldtmu41_returns_to_rf in 14 minutes and 18 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Containers Build
passed debian #574145

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  Build+Test
passed meson-arm64 #574154

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passed meson-armhf #574153

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passed meson-clang #574148

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passed meson-clover #574151

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passed meson-i386 #574155

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passed meson-main #574146

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passed meson-swr-glvnd #574147

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passed meson-vulkan #574152

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passed scons-llvm #574157

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passed scons-nollvm #574156

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passed scons-swr #574149

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passed scons-win64 #574150

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  Test
passed test-llvmpipe-gles2 1/4 #574158

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passed test-llvmpipe-gles2 2/4 #574159

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passed test-llvmpipe-gles2 3/4 #574160

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passed test-llvmpipe-gles2 4/4 #574161

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passed test-softpipe-gles2 1/4 #574162

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passed test-softpipe-gles2 2/4 #574163

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passed test-softpipe-gles2 3/4 #574164

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passed test-softpipe-gles2 4/4 #574165

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passed test-softpipe-gles3-limited #574166

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