OpConditionalEntryPointINTEL not handled by spirv-opt

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 6c6152b64fb1f14d84c7b55b56af32d151e6b372 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:

```spirv
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
```

Reproduce the failure at commit 6c6152b64fb1f14d84c7b55b56af32d151e6b372 with spirv-as piped into spirv-opt -Os, then inspect the disabled spirv-opt tests for SPV_INTEL_function_variants. Trace how OpConditionalEntryPointINTEL is handled outside function definitions; done means the repro no longer reports an unhandled opcode and the relevant tests can be enabled.

Written by the indexing model from the issue text.