[PATCH] [ARM64] Correct more bounds checks/diagnostics for arithmetic shift operands

[Tim Northover]

> The shifts are encoded into a single integer in such a way that 0x3f is the maximum value for the shift, if you add one that exceeds this then it overflows into the shift type field in such a way that can turn it into a different valid shift type, if it doesn't assert first.

OK, that makes sense for now. You should probably commit this one as
it is then, and we'll handle that combined operand separately.

Cheers.

Tim.

http://reviews.llvm.org/D3700