[SLP] Fix crash when matching associative reduction for integer min/max.

Associative reduction matcher in SLP begins with select instruction but when
it reached call to llvm.umax (or alike) via def-use chain the latter also matched
as UMax kind. The routine's later code assumes matched instruction to be a select
and thus it merely died on the first encountered cast that did not fit.

Differential Revision: https://reviews.llvm.org/D98432