Creating llvm-gcc-1200-08 branch

llvm-svn: 41088

When building casts, constant fold with ConstantFoldInstruction instead
of ConstantExpr::getCast.  This allows target-data driven constant folding
to happen.  In the testcase from PR1602, for example, this results in:

        %tmp26 = sub i32 %tmp232425, 1          

instead of:

        %tmp26 = sub i32 %tmp232425, ptrtoint (i32 (...)** inttoptr (i64 1 to i32 (...)**) to i32)

llvm-svn: 41071