The instructions emitted by the lowering operations require memory
operands to be aligned to 16 bytes. Since there is no support for
aligning memory operands in Subzero, do the arithmetic in registers for
now.

Add vector arithmetic to the arith crosstest. Pass the -mstackrealign
parameter to the crosstest clang so that llc code called back from
Subzero code (helper calls) doesn't assume that the stack is aligned at
the entry to the call.

BUG=none
R=jvoung@chromium.org, stichnot@chromium.org

Review URL: https://codereview.chromium.org/397833002