Subzero: Fix some legalization issues involving immediates.

Some lowering sequences were incorrectly allowing immediate operands in native instructions.  This includes 32-bit icmp, 64-bit icmp, select, switch, and 64-bit mul.

BUG= none
R=jvoung@chromium.org

Review URL: https://codereview.chromium.org/511543002
6 files changed