Use a memcpy so that type based alias analysis sees the change.

The other option would be to do something like

if (that.isSingleWord())
  VAL = that.VAL;
else
  pVal = that.pVal

This bug was causing 86TTI::getIntImmCost to be miscompiled in a LTO
bootstrap in stage2, causing the build of stage3 to fail.

LLVM is getting quiet good at exploiting this. Not sure if there is anything
a sanitizer could do to help