[#768] msp430x: Fix immediate addressing

[peroxyacyl]

This PR fixes renode/renode#768

Description

The way MSP430X handles immediate values in the instruction is complicated because of the 20-bit extension.
The immediate addressing is represented in the instructions as @PC+. The current implementation is written along with @Rn+. It is valid for 16-bit mode but not what is expected for 20-bit mode.

The current implementation for @PC+ here acts like:

1. get value at PC, which is the address of the immediate operand in the instruction
2. append the 4-bit address extension
3. increment PC by 2
4. access the operand by the 20-bit address
5. return the operand

Expected behaviour

The address extension should be appended after fetching the immediate operand word.

1. get value at PC, which is the address of the immediate operand in the instruction
2. access the operand by the 16-bit address
3. append the 4-bit address extension
4. increment PC by 2
5. return the 20-bit operand

[CLAassistant]

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