[NFC][SCEV] Recognize umin_seq when operand is zext'ed in zero-check

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

While it is not a very likely scenario, we probably should not expect
that instcombine already dropped such a redundant zext,
but handle directly. Moreover, perhaps there was no ZExtInst,
and SCEV somehow managed to  pull out said zext out of the SCEV expression.