CodeGen: Recreate old ivs with the original type

To avoid overflows we still use a larger type (i64) while calculating the value
of the old ivs.  However, we truncate the result to the type of the old iv when
providing it to the new code.

A corresponding test case is added to the polly test suite. Also, a failing test
case is fixed.

This fixes PR12311.

Contributed by: Tsingray Liu  <tsingrayliu@gmail.com>

llvm-svn: 153952