Subzero: Fix JumpTable lowering on x86-64.

The problem is that when switch lowering decides to use a JumpTable and the switch variable is i64 on x86-64, the lowering tries to movzx the i64 variable into an i32 variable, and the Movzx ctor asserts.

This happens when translating pnacl-llc.pexe, but luckily it is also triggered in the existing adv-switch.ll test.

BUG= none
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/1743133002 .