[NFC][SCEV] Recognize umin_seq when operand is zext'ed in umin but not in zero-check

zext(umin(x,y)) == umin(zext(x),zext(y))
zext(x) == 0  ->  x == 0

Extra leading zeros do not affect the result of comparison with zero,
nor do they matter for the unsigned min/max,
so we should not be dissuaded when we find a zero-extensions,
but instead we should just skip it.