[DAGCombiner] prevent infinite looping by truncating 'and' (PR40793)

This fold can occur during legalization, so it can fight with promotion
to the larger type. It apparently takes a special sequence and subtarget
to avoid more basic simplifications that would hide the problem.

But there's a bigger question raised here: why does distributeTruncateThroughAnd()
even exist? It duplicates functionality from a more minimal pattern that we
already have. But getting rid of this function requires some preliminary steps.

https://bugs.llvm.org/show_bug.cgi?id=40793

llvm-svn: 354594