bpf: Enable 32-bit subregister support for -mattr=+alu32

After all those preparation patches, now we could enable 32-bit subregister
support once -mattr=+alu32 specified.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325989