SelectionDAG: Add sext_inreg optimizations

v2: use dyn_cast
    fixup comments
v3: use cast

Reviewed-by: Matt Arsenault <arsenm2@gmail.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
R600: Add EG to FMA test

Reviewed-by: Tom Stellard <tom@stellard.net>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>
Mips: Only set divrem i64 to custom on 64bit

Reviewed-by: Daniel Sanders <daniel.sanders@imgtec.com>
Signed-off-by: Jan Vesely <jan.vesely@rutgers.edu>