Warn on 64-to-32 for source value of x bits where 64 >= x > 32.

The codepath already only works for source bits > target bits, it's just that
it was testing for the source expr bits to be exactly 64. This meant simple
cases (int i = x_long / 2) were missed & ended up under the general
-Wconversion warning, which a user might not have enabled.

llvm-svn: 154626