Implement scalarizeArithmetic() which extracts the components of the
input vectors, performs the operation with scalar instructions, and
builds the output vector component by component.

Fix the lowering of sdiv and srem.  These were previously emitting a
wrong instruction (cdq) for i8 and i16 inputs (needing cbw, cwd).

In the test_arith crosstest, mask the inputs to vector shift
operations to ensure that the shifts are in range.  Otherwise the
Subzero output is not identical to the llc output in some (undefined)
cases.

BUG=none
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/443203003