[SelectionDAG][AArch64][ARM] Remove setFlags call from DAGTypeLegalizer::SetPromotedInteger.

This was originally added to preserve FMF on SETCC. Unfortunately,
it also incorrectly preserves nuw/nsw on ADD/SUB in some cases.

There's also no guarantee the new opcode is even the same opcode
as the original node.

This patch removes the code and adds code to explicitly preserve
FMF flags in the SETCC promotion function.

The other test changes are from nuw/nsw not being preserved. I
believe for all these tests it was correct to preserve the flags,
so we need new code to preserve the flags when possible. I'll post
another patch for that since it's a riskier change.

This should unblock D150769.

Differential Revision: https://reviews.llvm.org/D151472