[DAGCombine] Basic combines for AVG nodes.

This adds very basic combines for AVG nodes, mostly for constant folding
and handling degenerate (zero) cases. The code performs mostly the same
transforms as visitMULHS, adjusted for AVG nodes.

Constant folding extends to a higher bitwidth and drops the lowest bit.
For undef nodes, `avg undef, x` is transformed to x.  There is also a
transform for `avgfloor x, 0` transforming to `shr x, 1`.

Differential Revision: https://reviews.llvm.org/D119559