R600: Add carry and borrow instructions. Use them to implement UADDO/USUBO

v2: tighten the sub64 tests
v3: rename to CARRY/BORROW
v4: fixup test cmdline
    add known bits computation
    use sign extend instead of sub 0,x
    better add test
v5: remove redundant break
    move lowering to separate functions
    fix comments

Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Reviewers: arsenm
llvm-svn: 234759