[SPIR-V] Use a helper class for most if/else branching

Simplifies emission of the blocks themselves (including inserting blocks
into the function's block list in the correct order), as well as phi after
the branching.

Also fixes 64bpp storing with blending in the fragment shader interlock
render backend implementation (had a typo that caused the high 32 bits to
overwrite the low ones).

src/xenia/gpu/spirv_builder.h

@@ -10,7 +10,10 @@
 #ifndef XENIA_GPU_SPIRV_BUILDER_H_
 #define XENIA_GPU_SPIRV_BUILDER_H_
 
+#include <optional>
+
 #include "third_party/glslang/SPIRV/SpvBuilder.h"
+#include "xenia/base/assert.h"
 
 namespace xe {
 namespace gpu {
@@ -42,6 +45,60 @@ class SpirvBuilder : public spv::Builder {
   spv::Id createTriBuiltinCall(spv::Id result_type, spv::Id builtins,
                                int entry_point, spv::Id operand1,
                                spv::Id operand2, spv::Id operand3);
+
+  // Helper to use for building nested control flow with if-then-else with
+  // additions over SpvBuilder::If.
+  class IfBuilder {
+   public:
+    IfBuilder(spv::Id condition, unsigned int control, SpirvBuilder& builder,
+              unsigned int thenWeight = 0, unsigned int elseWeight = 0);
+
+    ~IfBuilder() {
+#ifndef NDEBUG
+      assert_true(currentBranch == Branch::kMerge);
+#endif
+    }
+
+    void makeBeginElse(bool branchToMerge = true);
+    void makeEndIf(bool branchToMerge = true);
+
+    // If there's no then/else block that branches to the merge block, the phi
+    // parent is the header block - this simplifies then-only usage.
+    spv::Id getThenPhiParent() const { return thenPhiParent; }
+    spv::Id getElsePhiParent() const { return elsePhiParent; }
+
+    spv::Id createMergePhi(spv::Id then_variable, spv::Id else_variable) const;
+
+   private:
+    enum class Branch {
+      kThen,
+      kElse,
+      kMerge,
+    };
+
+    IfBuilder(const IfBuilder& ifBuilder) = delete;
+    IfBuilder& operator=(const IfBuilder& ifBuilder) = delete;
+
+    SpirvBuilder& builder;
+    spv::Id condition;
+    unsigned int control;
+    unsigned int thenWeight;
+    unsigned int elseWeight;
+
+    spv::Function& function;
+
+    spv::Block* headerBlock;
+    spv::Block* thenBlock;
+    spv::Block* elseBlock;
+    spv::Block* mergeBlock;
+
+    spv::Id thenPhiParent;
+    spv::Id elsePhiParent;
+
+#ifndef NDEBUG
+    Branch currentBranch = Branch::kThen;
+#endif
+  };
 };
 
 }  // namespace gpu