[GlobalIsel][X86] Update legalization of G_ADD/G_SUB

Replace the legacy legalizers for G_ADD/G_SUB with a pattern that should work for the full mixture of scalar/vector types we need to support for most opcodes.

If we can decide on this as a suitable pattern we can reuse it for the other op types, but G_ADD/G_SUB are the only ones that have decent legalizer test coverage at this stage.

Differential Revision: https://reviews.llvm.org/D151878