Jit: Check MSR state in BLR optimization

When we execute a JIT block, we have to make sure that both the PC and
the DR/IR bits of MSR are the same as they were when the block was
compiled. When jumping to a block from the dispatcher, this is done in
the way you would expect: By checking the PC and the relevant MSR bits.
However, when returning to a block using the BLR optimization, we only
check the PC. Checking the MSR bits is done by instead resetting the
stack when the MSR changes, making PC checks afterwards fail.

Except... We were only resetting the stack on rfi instructions. There
are actually many more ways for the MSR to change, and we weren't
covering those at all. I looked into resetting the stack on all of them,
but it would be pretty cumbersome both in terms of writing the code and
in terms of how often at runtime we'd have to reset the stack, so I
think the better option would be to check the MSR bits along with the
PC. That's what this commit implements.

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