SPIR-V: Initialize undefined variables with 0 (#3526)

* SPIR-V: Initialize undefined variables with a value

Changes undefined values on spir-v shaders (caused by phi nodes) to be initialized instead of truly undefined.

Fixes an issue with NVIDIA gpus seemingly not liking when a variable is _potentially_ undefined. Not sure about the details at the moment.

Fixes:
- Tilt shift blur effect in Link's Awakening (bottom of the screen)
- Potentially block flickering on newer NVIDIA gpus in Splatoon 2? Needs testing.

Testing is welcome.

* Update Ryujinx.Graphics.Shader/CodeGen/Spirv/CodeGenContext.cs

Co-authored-by: gdkchan <gab.dark.100@gmail.com>

Co-authored-by: gdkchan <gab.dark.100@gmail.com>

Ryujinx.Graphics.Shader/CodeGen/Spirv/CodeGenContext.cs

@@ -234,7 +234,7 @@ namespace Ryujinx.Graphics.Shader.CodeGen.Spirv
                     IrOperandType.Constant => GetConstant(type, operand),
                     IrOperandType.ConstantBuffer => GetConstantBuffer(type, operand),
                     IrOperandType.LocalVariable => GetLocal(type, operand),
-                    IrOperandType.Undefined => Undef(GetType(type)),
+                    IrOperandType.Undefined => Constant(GetType(type), 0),
                     _ => throw new ArgumentException($"Invalid operand type \"{operand.Type}\".")
                 };
             }