Reduce a lot of code duplication by implementing

ConstantExpr::getWithOperandReplaced and ConstantExpr::replaceUsesOfWithOnConstant
 in terms of ConstantExpr::getWithOperands.  While we're at it,
make sure that ConstantExpr::getWithOperands covers all instructions: it was
missing insert/extractvalue.  

llvm-svn: 149076