Revert "[DAG] SimplifyDemandedBits - if we're only demanding the signbit, a...

Revert "[DAG] SimplifyDemandedBits - if we're only demanding the signbit, a SMIN/SMAX node can be simplified to a OR/AND node respectively."

This reverts commit 54d663d5, which breaks the test CodeGen/SystemZ/ctpop-01.ll for stage2-ubsan check (see https://lab.llvm.org/buildbot/#/builders/85/builds/18410)

I manually confirmed that the test had been passing immediately prior to that commit
(BUILDBOT_REVISION=4772c66c llvm-zorg/zorg/buildbot/builders/sanitizers/buildbot_bootstrap_ubsan.sh)