bpf: Support i32 in getScalarShiftAmountTy method

getScalarShiftAmount method should be implemented for eBPF backend to make
sure shift amount could still get correct type once 32-bit subregisters
support are enabled.

Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
Reviewed-by: Yonghong Song <yhs@fb.com>
llvm-svn: 325986