[SDAG][AArch64] Boolean and/or reduce to umax/min reduce (PR41635)

This addresses one half of https://bugs.llvm.org/show_bug.cgi?id=41635
by combining a VECREDUCE_AND/OR into VECREDUCE_UMIN/UMAX (if latter is
legal but former is not) for zero-or-all-ones boolean reductions (which
are detected based on sign bits).

Differential Revision: https://reviews.llvm.org/D61398

llvm-svn: 360054