[llvm] [CodeGen] Do not emit TRAP for `unreachable` after `@llvm.trap` (PR #94570)

[Igor Kudrin via llvm-commits]

igorkudrin wrote:

> Should there be a similar check somewhere in FastISel?

As for now, `FastISel::selectOperator()` for `Instruction::Unreachable` with `TM.Options.TrapUnreachable == true` returns `false`. I added a test to explicitly check that the behavior for `-fast-isel` is the same as with other options.

https://github.com/llvm/llvm-project/pull/94570