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

[via llvm-commits]

urosbericsyrmia wrote:

> @urosbericsyrmia it may be useful to check why Mips FastISel creates such a pattern while other targets don't:) Leave it to you:)
Great observation. Originally, I was able to reproduce the error with `llc -O0 --relocation-model=pic`. I might come back sometime to see why FastISel creates such pattern.


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