Revert "[GlobalIsel][X86] Legalize G_BSWAP"

This reverts commit 5cafecf9.

Buildbots are not happy with the patch.

Lots of crashes and assertion failures such as

  llvm::LegalizeRuleSet &llvm::LegalizerInfo::getActionDefinitionsBuilder(
  std::initializer_list<unsigned int>): Assertion `Opcodes.size() >= 2 &&
  "Initializer list must have at least two opcodes"' failed.