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

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@154626 91177308-0d34-0410-b5e6-96231b3b80d8