Implement folding of expressions like 'uint cast (int* getelementptr (int*

Implement folding of expressions like 'uint cast (int* getelementptr (int*
null, uint 1) to uint)' to a constant integer.  We can only do this with
primitive LLVM types, because other types have target-specific sizes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14837 91177308-0d34-0410-b5e6-96231b3b80d8