shader/conversion: Implement byte selector in I2F

I2F's byte selector is used to choose what bytes to convert to float.
e.g. if the input is 0xaabbccdd and the selector is ".B3" it will
convert 0xaa. The default (when it's not shown in nvdisasm) is ".B0", in
that example the default would convert 0xdd to float.

src/video_core/shader/decode/conversion.cpp

@@ -63,12 +63,11 @@ u32 ShaderIR::DecodeConversion(NodeBlock& bb, u32 pc) {
     case OpCode::Id::I2F_R:
     case OpCode::Id::I2F_C:
     case OpCode::Id::I2F_IMM: {
-        UNIMPLEMENTED_IF(instr.conversion.int_src.selector != 0);
         UNIMPLEMENTED_IF(instr.conversion.dst_size == Register::Size::Long);
         UNIMPLEMENTED_IF_MSG(instr.generates_cc,
                              "Condition codes generation in I2F is not implemented");
 
-        Node value = [&]() {
+        Node value = [&] {
             switch (opcode->get().GetId()) {
             case OpCode::Id::I2F_R:
                 return GetRegister(instr.gpr20);
@@ -81,7 +80,19 @@ u32 ShaderIR::DecodeConversion(NodeBlock& bb, u32 pc) {
                 return Immediate(0);
             }
         }();
+
         const bool input_signed = instr.conversion.is_input_signed;
+
+        if (instr.conversion.src_size == Register::Size::Byte) {
+            const u32 offset = static_cast<u32>(instr.conversion.int_src.selector) * 8;
+            if (offset > 0) {
+                value = SignedOperation(OperationCode::ILogicalShiftRight, input_signed,
+                                        std::move(value), Immediate(offset));
+            }
+        } else {
+            UNIMPLEMENTED_IF(instr.conversion.int_src.selector != 0);
+        }
+
         value = ConvertIntegerSize(value, instr.conversion.src_size, input_signed);
         value = GetOperandAbsNegInteger(value, instr.conversion.abs_a, false, input_signed);
         value = SignedOperation(OperationCode::FCastInteger, input_signed, PRECISE, value);