[InstCombine] fold (trunc (X>>C1)) << C to shift+mask directly

This is no-externally-visible-functional-difference-intended.
That is, the test diffs show identical instructions other than
name changes (those are included specifically to verify the logic).

The existing transforms created extra instructions and relied
on subsequent folds to get to the final result, but that could
conflict with other transforms like the proposed D110170 (and
caused that patch to be reverted twice so far because of infinite
combine loops).