[DAGCombiner] Don't peek through truncates of shift amounts in takeInexpensiveLog2. (#126957)

Shift amounts in SelectionDAG don't have to match the result type
of the shift. SelectionDAGBuilder will aggressively truncate shift
amounts to the target's preferred type. This may result in a zero extend
that existed in IR being removed.
    
If we look through a truncate here, we can't guarantee the upper bits
of the truncate input are zero. There may have been a zext that was
removed. Unfortunately, this regresses tests where no truncate was
involved. The only way I can think to fix this is to add an assertzext
when SelectionDAGBuilder truncates a shift amount or remove the
early truncation of shift amounts from SelectionDAGBuilder all together.
    
Fixes #126889.

0 files changed, 0 insertions, 0 deletions