OpConditionalEntryPointINTEL not handled by spirv-opt

[Keenuts]

This is to track some disabled tests in spirv-opt.
The support for SPV_INTEL_function_variants being quite recent, seems like some part are missing in spirv-opt.

Can repro at 6c6152b by running:

./build/tools/spirv-as /tmp/1.spv -o - | ./build/tools/spirv-opt -Os - -o /tmp/1
error: line 0: Unhandled inst type (opcode: 6249) found outside function definition.

with the following code:

               OpCapability Shader
               OpCapability Geometry
               OpCapability SpecConditionalINTEL
               OpExtension "SPV_INTEL_function_variants"
               OpMemoryModel Logical GLSL450
               OpConditionalEntryPointINTEL %false Fragment %ps_main "ps_main" %in_var_POSITION %out_var_SV_Target
               OpExecutionMode %ps_main OriginUpperLeft
               OpSource HLSL 660
               OpName %in_var_POSITION "in.var.POSITION"
               OpName %out_var_SV_Target "out.var.SV_Target"
               OpName %ps_main "ps_main"
               OpDecorate %in_var_POSITION Location 0
               OpDecorate %out_var_SV_Target Location 0
       %bool = OpTypeBool
      %false = OpSpecConstantFalse %bool
      %float = OpTypeFloat 32
    %v4float = OpTypeVector %float 4
%_ptr_Input_v4float = OpTypePointer Input %v4float
%_ptr_Output_v4float = OpTypePointer Output %v4float
       %void = OpTypeVoid
          %9 = OpTypeFunction %void
%in_var_POSITION = OpVariable %_ptr_Input_v4float Input
%out_var_SV_Target = OpVariable %_ptr_Output_v4float Output
    %ps_main = OpFunction %void None %9
         %10 = OpLabel
         %11 = OpLoad %v4float %in_var_POSITION
               OpStore %out_var_SV_Target %11
               OpReturn
               OpFunctionEnd