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spirv: fix emitting switch cases that directly jump to the merge block

Samuel Pitoiset requested to merge hakzsam/mesa:spirv_fix_target_merge_block into master

As shown in the valid SPIR-V below, if one switch case statement directly jumps to the merge block, it has no branches at all and we have to reset the fall variable. Otherwise, it creates an unintentional fallthrough.

       OpSelectionMerge %97 None
       OpSwitch %96 %97 1 %99 2 %100
%100 = OpLabel
%102 = OpAccessChain %_ptr_StorageBuffer_v4float %86 %uint_0 %uint_37
%103 = OpLoad %v4float %102
%104 = OpBitcast %v4uint %103
%105 = OpCompositeExtract %uint %104 0
%106 = OpShiftLeftLogical %uint %105 %uint_1
       OpBranch %97
 %99 = OpLabel
       OpBranch %97
 %97 = OpLabel
%107 = OpPhi %uint %uint_4 %75 %uint_5 %99 %106 %100

This fixes serious corruption in Horizon Zero Dawn.

Closes: #3460 (closed) Cc: mesa-stable Signed-off-by: Samuel Pitoiset samuel.pitoiset@gmail.com

Edited by Samuel Pitoiset

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