[CodeGen] Canonicalise adds/subs of i1 vectors using XOR

When calling SelectionDAG::getNode() to create an ADD or SUB
of two vectors with i1 element types we can canonicalise this
to use XOR instead, where 1+1 is treated as wrapping around
to 0 and 0-1 wraps to 1.

I've added the following tests for SVE targets:

  CodeGen/AArch64/sve-pred-arith.ll

and modified some X86 tests to reflect the much simpler codegen
required.

Differential Revision: https://reviews.llvm.org/D97276