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
87dbcd8865