[GPU/D3D12] Memexport from anywhere in control flow + 8/16bpp memexport

There's no limit on the number of memory exports in a shader on the real
Xenos, and exports can be done anywhere, including in loops. Now, instead
of deferring the exports to the end of the shader, and assuming that export
allocs are executed only once, Xenia flushes exports when it reaches an
alloc (allocs terminate memory exports on Xenos, as well as individual ALU
instructions with `serialize`, but not handling this case for simplicity,
it's only truly mandatory to flush memory exports before starting a new
one), the end of the shader, or a pixel with outstanding exports is killed.

To know which eM# registers need to be flushed to the memory, traversing
the successors of each exec potentially writing any eM#, and specifying
that certain eM# registers might have potentially been written before each
reached control flow instruction, until a flush point or the end of the
shader is reached.

Also, some games export to sub-32bpp formats. These are now supported via
atomic AND clearing the bits of the dword to replace followed by an atomic
OR inserting the new byte/short.

src/xenia/gpu/shader_translator.h

@@ -127,8 +127,10 @@ class ShaderTranslator {
   virtual void ProcessReturnInstruction(const ParsedReturnInstruction& instr) {}
   // Handles translation for jump instructions.
   virtual void ProcessJumpInstruction(const ParsedJumpInstruction& instr) {}
-  // Handles translation for alloc instructions.
-  virtual void ProcessAllocInstruction(const ParsedAllocInstruction& instr) {}
+  // Handles translation for alloc instructions. Memory exports for eM#
+  // indicated by export_eM must be performed, regardless of the alloc type.
+  virtual void ProcessAllocInstruction(const ParsedAllocInstruction& instr,
+                                       uint8_t export_eM) {}
 
   // Handles translation for vertex fetch instructions.
   virtual void ProcessVertexFetchInstruction(
@@ -137,7 +139,13 @@ class ShaderTranslator {
   virtual void ProcessTextureFetchInstruction(
       const ParsedTextureFetchInstruction& instr) {}
   // Handles translation for ALU instructions.
-  virtual void ProcessAluInstruction(const ParsedAluInstruction& instr) {}
+  // memexport_eM_potentially_written_before needs to be handled by `kill`
+  // instruction to make sure memory exports for the eM# writes earlier in
+  // previous execs and the current exec are done before the invocation becomes
+  // inactive.
+  virtual void ProcessAluInstruction(
+      const ParsedAluInstruction& instr,
+      uint8_t memexport_eM_potentially_written_before) {}
 
  private:
   void TranslateControlFlowInstruction(const ucode::ControlFlowInstruction& cf);