shader: support DS_READ_U16_D16_HI (#356)

Decode the high-half D16 LDS read and route it through the existing unsigned-short load lowering. Represent the destination as a high-half partial write so the low 16 bits of the VGPR remain intact. Add captured-instruction decode coverage and a Vulkan execution regression that verifies the preserved half.

Co-authored-by: SueryRD <the.ai.tester2@gmail.com>

src/graphics/shader/recompiler/frontend/decode/ShaderDecoder.h

@@ -497,6 +497,7 @@ enum class Opcode {
 	DS_READ_I16,
 	DS_READ_U16,
 	DS_READ_U16_D16,
+	DS_READ_U16_D16_HI,
 	DS_READ2_B32,
 	DS_READ2ST64_B32,
 	DS_READ_B32,