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

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

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.

v2: Changed the code to skip the entire if-block instead of resetting
    the fallthrough variable.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3460
Cc: mesa-stable
Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6590>