This Ableton tutorial from the Learn Live series walks through four closely related note-editing techniques applied to a drum loop in Live 12's MIDI Note View: slicing to the grid, dividing into equal parts, manually cutting at a chosen point, and joining notes back together.
Drawing a long note on the hi-hat pad and pressing Command E (Mac) or Control E (Windows) slices it to the current grid setting. Changing the grid size and re-slicing lets you create tighter subdivisions and drum rolls quickly.
For equal-division slicing on the snare, hold Option + E and drag up or down on Mac, or hold E + Ctrl on Windows. Manual cuts work by holding E and clicking exactly where you want to cut, giving you freeform control over any note or selection. Selected notes can be rejoined with Command + J (Mac) or Control + J (Windows).