Fix pc_array bounds check to use elements instead of bytes

pc_array_size and kPcArrayMaxSize appear to be measured in elements, not
bytes, so we shouldn't multiply idx by sizeof(uptr) in this bounds
check.  32-bit Chrome was tripping this assertion because it has 64
million coverage points. I don't think it's worth adding a test that has
that many coverage points.

llvm-svn: 292955