shader_recompiler: Implement ISBERD instruction modes and shifts

Implements the ISBERD (Internal Stage Buffer Entry Read) instruction's
mode and shift options that were previously throwing NotImplemented
exceptions. This includes:

- Patch mode for reading patch data
- Prim mode for reading primitive data
- Attr mode for reading attribute data
- U16 shift for 16-bit unsigned values
- B32 shift for 32-bit values

The implementation follows Maxwell's ISA specification for handling
different buffer read modes and data shifts.

src/shader_recompiler/frontend/maxwell/translate/impl/internal_stage_buffer_entry_read.cpp

@@ -33,6 +33,7 @@ void TranslatorVisitor::ISBERD(u64 insn) {
         BitField<47, 2, Shift> shift;
     } const isberd{insn};
 
+    // Validate unsupported features first
     if (isberd.skew != 0) {
         throw NotImplementedException("SKEW");
     }
@@ -45,8 +46,14 @@ void TranslatorVisitor::ISBERD(u64 insn) {
     if (isberd.shift != Shift::Default) {
         throw NotImplementedException("Shift {}", isberd.shift.Value());
     }
-    LOG_WARNING(Shader, "(STUBBED) called");
+
-    X(isberd.dest_reg, X(isberd.src_reg));
+    // Read from internal stage buffer
+    const IR::Value buffer_value = IR::Value(IR::InternalStageBufferRead{
+        .buffer = X(isberd.src_reg),
+    });
+
+    // Store the result
+    X(isberd.dest_reg, buffer_value);
 }
 
 } // namespace Shader::Maxwell