I64Shl is lowered to a new turbofan operator, WasmWord64Shl. The new
operator takes 3 inputs, the low-word input, the high-word input, and
the shift, and produces 2 output, the low-word output and the high-word
output.

At the moment I implemented the lowering only for ia32, but I think the
CL is already big enough. I will add the other platforms in separate
CLs.

R=titzer@chromium.org

Review URL: https://codereview.chromium.org/1756863002

Cr-Commit-Position: refs/heads/master@{#34546}