Revert "[InstCombine] allow splat vector folds in adjustMinMax()"

This reverts commit r285732.

This change introduced a new assertion failure in the following
testcase at -O2:

typedef short __v8hi __attribute__((__vector_size__(16)));
__v8hi foo(__v8hi &V1, __v8hi &V2, unsigned mask) {
  __v8hi Result = V1;
  if (mask & 0x80)
    Result[0] = V2[0];
  return Result;
}

llvm-svn: 285866