[llvm] [MIPS] Use generic isBlockOnlyReachableByFallthrough (PR #80799)

[Fangrui Song via llvm-commits]

MaskRay wrote:

> This seems to be a more elegant solution to the problem. However, I do still think adding the test from @urosbericsyrmia's original PR (#80666) would be desirable.
> 
> Jovan

The problem appears FastISel specific (`llc -O0 -fast-isel=0` does not reproduce), and the change to `llvm/test/CodeGen/Mips/Fast-ISel/pr40325.ll` reflects the bug fix. I personally feel that this is enough, but I don't mind #80666 is changed to reflect a better test :)

@urosbericsyrmia it may be useful to check why Mips FastISel creates such a pattern while other targets don't:) Leave it to you:)

https://github.com/llvm/llvm-project/pull/80799