[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/vulkan/vulkan_command_processor.cc

@@ -2175,7 +2175,7 @@ bool VulkanCommandProcessor::IssueDraw(xenos::PrimitiveType prim_type,
     return false;
   }
   pipeline_cache_->AnalyzeShaderUcode(*vertex_shader);
-  bool memexport_used_vertex = vertex_shader->is_valid_memexport_used();
+  bool memexport_used_vertex = vertex_shader->memexport_eM_written();
 
   // Pixel shader analysis.
   bool primitive_polygonal = draw_util::IsPrimitivePolygonal(regs);