[DAG] Fix shift amount limit in SimplifyDemandedBits trunc(shift(x,c)) to truncated bitwidth

We lost this in D56387/rG69bc0990a9181e6eb86228276d2f59435a7fae67 - where I got the src/dst bitwidths mixed up and assumed getValidShiftAmountConstant would catch it.

Patch by @craig.topper - confirmed by @Carrot that it fixes PR49162