Regression: incorrect warnings claiming shaders aren't writing to attachments beyond the first one.

**Environment:**
- OS: Windows 10
- GPU and driver version: Quadro 6000 Ada, 522.22
- SDK or header version if building from repo: 1.3.280.0
- Options enabled (synchronization, best practices, etc.):

**Describe the Issue**

Starting in SDK 1.3.275.0 (and not occurring in 1.3.268.0) I now get incorrect validation warnings for attachments that the validation thinks aren't getting written by shaders.

```
WARNING : VALIDATION - Message Id Number: 1203141749 | Message Id Name: Undefined-Value-ShaderInputNotProduced
Validation Warning: [ Undefined-Value-ShaderInputNotProduced ] Object 0: handle = 0xf39fcf00000020fb, type = VK_OBJECT_TYPE_SHADER_MODULE; | MessageID = 0x47b67c75 | vkCreateGraphicsPipelines(): pCreateInfos[0] Attachment 1 not written by fragment shader; undefined values will be written to attachment
```

The below simple shader will produce the warning
```
SourceExtension "GL_EXT_nonuniform_qualifier"
SourceExtension "GL_EXT_scalar_block_layout"
SourceExtension "GL_GOOGLE_cpp_style_line_directive"
SourceExtension "GL_GOOGLE_include_directive"
Name 10 "main"
Name 29 "fragColor"
ModuleProcessed "auto-map-bindings"
ModuleProcessed "auto-map-locations"
ModuleProcessed "client vulkan100"
ModuleProcessed "target-env spirv1.3"
ModuleProcessed "target-env vulkan1.1"
ModuleProcessed "entry-point main"
ModuleProcessed "client vulkan100"
ModuleProcessed "target-env spirv1.3"
ModuleProcessed "target-env vulkan1.1"
ModuleProcessed "entry-point main"
Decorate 29(fragColor) Location 0
8: TypeVoid
9: TypeFunction 8
12: TypeFloat 32
13: TypeVector 12(float) 4
23: 12(float) Constant 1065353216
24: 13(fvec4) ConstantComposite 23 23 23 23
25: TypeInt 32 0
26: 25(int) Constant 2
27: TypeArray 13(fvec4) 26
28: TypePointer Output 27
29(fragColor): 28(ptr) Variable Output
30: TypeInt 32 1
31: 30(int) Constant 0
35: TypePointer Output 13(fvec4)
37: 30(int) Constant 1
Line 1 6 11
10(main): 8 Function None 9
NoLine
11: Label
Line 1 9 0
36: 35(ptr) AccessChain 29(fragColor) 31
Store 36 24
Line 1 10 0
41: 35(ptr) AccessChain 29(fragColor) 37
Store 41 24
Return
FunctionEnd
```

**Expected behavior**

This isn't an warning, the shader is correctly writing to that attachment.

**Valid Usage ID**

```
WARNING : VALIDATION - Message Id Number: 1203141749 | Message Id Name: Undefined-Value-ShaderInputNotProduced
Validation Warning: [ Undefined-Value-ShaderInputNotProduced ] Object 0: handle = 0xf39fcf00000020fb, type = VK_OBJECT_TYPE_SHADER_MODULE; | MessageID = 0x47b67c75 | vkCreateGraphicsPipelines(): pCreateInfos[0] Attachment 1 not written by fragment shader; undefined values will be written to attachment
```