[ISel] Port AArch64 HADD and RHADD to ISel

This ports the aarch64 combines for HADD and RHADD over to DAG combine,
so that they can be used in more architectures (notably MVE in a
followup patch). They are renamed to AVGFLOOR and AVGCEIL in the
process, to avoid confusion with instructions such as X86 hadd. The code
was also rewritten slightly to remove the AArch64 idiosyncrasies.

The general pattern for a AVGFLOORS is
  %xe = sext i8 %x to i32
  %ye = sext i8 %y to i32
  %a = add i32 %xe, %ye
  %r = lshr i32 %a, 1
  %t = trunc i32 %r to i8

An AVGFLOORU is equivalent with zext. Because of the truncate
lshr==ashr, as the top bits are not demanded. An AVGCEIL also includes
an extra rounding, so includes an extra add of 1.

Differential Revision: https://reviews.llvm.org/D106237