Update the ISA after instruction decode

[c722996]

For decoding ETM with thumb mode, we should also update the m_last_IS value from the instruction decoder.
Otherwise, the decoded ISA might be incorrect when handling the later context/atom element.
It might reflect an incorrect A32/T32 ISA.

[c722996]

Friendly ping.

[mikel-armbb]

This certainly requires further investigation.

The solution provided may not work in a speculative trace environment as the m_last_IS var is only used to update the context element at the time the single context packet is seen and before it is added to the element stack.