Subzero: Fix an Om1 crash from memset lowering.

With a certain combination of memset arguments, legalizeToReg() is called but its result is unused.  Om1 register allocation does not like this because it sees an infinite-weight variable with a definition but no uses.  The simplest fix is to add a fake use.

The problem shows up when building spec2k with -Om1.

BUG= none
R=ascull@google.com

Review URL: https://codereview.chromium.org/1272823004.