For PR1205:

Implement review feedback:
1. Use new APInt::RoundDoubleToAPInt interface to specify the bit width so
   that we don't have to truncate or extend in constant folding.
2. Fix a pasteo in SDiv that prevented a check for overflow.
3. Fix the shift operators: undef happens when the shift amount is equal
   to the bitwidth.

llvm-svn: 34677