[SLP] Fix crash in reduction for integer min/max

The SCEV commit b46c085d [NFCI] SCEVExpander:
    emit intrinsics for integral {u,s}{min,max} SCEV expressions
seems to reveal a new crash in SLPVectorizer.
SLP crashes expecting a SelectInst as an externally used value
but umin() call is found.

The patch relaxes the assumption to make the IR flag propagation safe.

Reviewed By: spatel

Differential Revision: https://reviews.llvm.org/D99328