The low-level mul instruction may produce results in a register pair where one register is the explicit dest of the instruction, and the other register is defined through a FakeDef.  If the FakeDef portion is ultimately unused, the FakeDef gets dead-code eliminated, and the register allocator doesn't know that the mul instruction affects the other register.

On x86, this can silently produce incorrect code.  On ARM, the emitter complains that the explicitly represented second dest variable does not have a register.

The fix is to add a FakeUse of the FakeDef'd register.  Unfortunately, this prevents the low-level mul instruction from ever being dead-code eliminated, but that's probably OK because it should have been eliminated at the high level.

BUG= none
R=eholk@chromium.org

Review URL: https://codereview.chromium.org/1678523002 .