[SDAG] Add SDNode/SDValue getConstantOperandAPInt helper. NFCI.

[SDAG] Add SDNode/SDValue getConstantOperandAPInt helper. NFCI.

We already have the getConstantOperandVal helper which returns a uint64_t, but along comes the fuzzer and inserts a i128 -1 constant or something and the whole thing asserts.......

I've updated a few obvious cases, and tried to make use of the const reference where possible, but there's more to do. A number of existing oss-fuzz tickets should be fixed if we start using APInt and perform value clamping where necessary.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@352961 91177308-0d34-0410-b5e6-96231b3b80d8