[CodeGen] Fix nullptr crash in tryConvertSVEWideCompare

When the input to a wide compare instruction is a DUP or SPLAT_VECTOR
node we should deal with cases where the DUP/SPLAT_VECTOR input
operand is not an immediate value. I've fixed the code to return
SDValue() in such cases and added a couple of tests - one each to
represent the signed and unsigned cases.

Differential Revision: https://reviews.llvm.org/D81167