[SDAG] improve variable names; NFC

The version of FoldConstantArithmetic() that takes arbitrary nodes
was confusingly naming those nodes as constants when they might
not be; also "Cst" reads like "Cast".


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352884 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed