[DAGCombiner] widen any_ext of popcount based on target support

This enhances D69127 (rGe6c145e0)
to handle the looser "any_extend" cast in addition to zext.

This is a prerequisite step for canonicalizing in the other direction
(narrow the popcount) in IR - PR43688:
https://bugs.llvm.org/show_bug.cgi?id=43688