dead_code_elimination_pass: Remove unreachable Phi arguments

src/shader_recompiler/frontend/ir/microinstruction.cpp

@@ -326,6 +326,11 @@ void Inst::AddPhiOperand(Block* predecessor, const Value& value) {
     phi_args.emplace_back(predecessor, value);
 }
 
+void Inst::ErasePhiOperand(size_t index) {
+    const auto operand_it{phi_args.begin() + static_cast<ptrdiff_t>(index)};
+    phi_args.erase(operand_it);
+}
+
 void Inst::OrderPhiArgs() {
     if (op != Opcode::Phi) {
         throw LogicError("{} is not a Phi instruction", op);

src/shader_recompiler/frontend/ir/value.h

@@ -179,9 +179,13 @@ public:
 
     /// Get a pointer to the block of a phi argument.
     [[nodiscard]] Block* PhiBlock(size_t index) const;
+
     /// Add phi operand to a phi instruction.
     void AddPhiOperand(Block* predecessor, const Value& value);
 
+    // Erase the phi operand at the given index.
+    void ErasePhiOperand(size_t index);
+
     /// Orders the Phi arguments from farthest away to nearest.
     void OrderPhiArgs();