Refactor several ConstantExpr::getXXX calls with ConstantInt arguments

Refactor several ConstantExpr::getXXX calls with ConstantInt arguments
using the facilities of APInt. While this duplicates a tiny fraction of
the constant folding code, it also makes the code easier to read and
avoids large ConstantExpr overhead for simple, known computations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@35335 91177308-0d34-0410-b5e6-96231b3b80d8