[SDAG] fix bug in ComputeNumSignBits of target constant

The loop below the changed line assumes that the element
width of the target constant is the same as the element
width of the loaded value, but that is not always true.

We could try harder to do some kind of min/max calc even
if the sizes don't match, but that can be another patch
if needed. This fixes #53401 (miscompile) and does not
change the motivating cases added when this analysis
was introduced:
ad298f86