For certain cases of inlined memset lowering, the 8-bit value wasn't being properly spread/replicated into the 32-bit immediate to be stored.

Specifically, if the 8-bit value is between -128 and -1 (i.e. 0x80 to 0xff), the spread value would be something like 0xffffff80 instead of 0x80808080.

BUG= b/30502279
R=jpp@chromium.org

Review URL: https://codereview.chromium.org/2215553002 .