shader/decode: Fix constant buffer offsets

Some instances were using cbuf34.offset instead of cbuf34.GetOffset().
This returned the an invalid offset. Address those instances and rename
offset to "shifted_offset" to avoid future bugs.

src/video_core/engines/shader_bytecode.h

@@ -1657,11 +1657,11 @@ union Instruction {
     } xmad;
 
     union {
-        BitField<20, 14, u64> offset;
+        BitField<20, 14, u64> shifted_offset;
         BitField<34, 5, u64> index;
 
         u64 GetOffset() const {
-            return offset * 4;
+            return shifted_offset * 4;
         }
     } cbuf34;
 

src/video_core/shader/decode/arithmetic_integer.cpp

@@ -166,13 +166,13 @@ u32 ShaderIR::DecodeArithmeticInteger(NodeBlock& bb, u32 pc) {
         const auto [op_rhs, test] = [&]() -> std::pair<Node, Node> {
             switch (opcode->get().GetId()) {
             case OpCode::Id::ICMP_CR:
-                return {GetConstBuffer(instr.cbuf34.index, instr.cbuf34.offset),
+                return {GetConstBuffer(instr.cbuf34.index, instr.cbuf34.GetOffset()),
                         GetRegister(instr.gpr39)};
             case OpCode::Id::ICMP_R:
                 return {GetRegister(instr.gpr20), GetRegister(instr.gpr39)};
             case OpCode::Id::ICMP_RC:
                 return {GetRegister(instr.gpr39),
-                        GetConstBuffer(instr.cbuf34.index, instr.cbuf34.offset)};
+                        GetConstBuffer(instr.cbuf34.index, instr.cbuf34.GetOffset())};
             case OpCode::Id::ICMP_IMM:
                 return {Immediate(instr.alu.GetSignedImm20_20()), GetRegister(instr.gpr39)};
             default:

src/video_core/shader/decode/bfi.cpp

@@ -21,7 +21,7 @@ u32 ShaderIR::DecodeBfi(NodeBlock& bb, u32 pc) {
         switch (opcode->get().GetId()) {
         case OpCode::Id::BFI_RC:
             return {GetRegister(instr.gpr39),
-                    GetConstBuffer(instr.cbuf34.index, instr.cbuf34.offset)};
+                    GetConstBuffer(instr.cbuf34.index, instr.cbuf34.GetOffset())};
         case OpCode::Id::BFI_IMM_R:
             return {Immediate(instr.alu.GetSignedImm20_20()), GetRegister(instr.gpr39)};
         default: