[ValueTracking] matchSelectPattern needs to be more careful around FP

matchSelectPattern attempts to see through casts which mask min/max
patterns from being more obvious.  Under certain circumstances, it would
misidentify a sequence of instructions as a min/max because it assumed
that folding casts would preserve the result.  This is not the case for
floating point <-> integer casts.

This fixes PR27575.

llvm-svn: 268086