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

[Jovan Dmitrović via llvm-commits]

jdmitrovic-syrmia 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.

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