[llvm] [X86] Don't always seperate conditions in `(br (and/or cond0, cond1))` into seperate branches (PR #81689)

[via llvm-commits]

goldsteinn wrote:

> > Some initial comments - but I can't see why so much logic is in x86 directly. The keepJumpConditionsTogether implementation appears very generic, and I'd expect it to be in DAGBuilder and x86 just has a simple TLI flag callback to enable it.
> 
> Fair enough. Ill put the code somewhere generic and make it a call in.

Have moved code to `SelectionDAG` w/ targets providing the tuning params. I kept the special case for `(a == b && c == d)` in x86 as not sure if thats really general.

https://github.com/llvm/llvm-project/pull/81689