[MIPS] Use generic isBlockOnlyReachableByFallthrough (#80799)

FastISel may create a redundant BGTZ terminal which fallthroughes.
```
  BGTZ %2:gpr32, %bb.1, implicit-def $at

bb.1.bb1:
; predecessors: %bb.0
```

The `!I->isBarrier()` check in
MipsAsmPrinter::isBlockOnlyReachableByFallthrough
will incorrectly not print a label, leading to a `Undefined temporary
symbol `
error when we try assembling the output assembly file. See the updated
`Fast-ISel/pr40325.ll` and
https://github.com/rust-lang/rust/issues/108835

In addition, the `SwitchInst` condition is too conservative and prints
many unneeded labels (see the updated tests).

Just use the generic isBlockOnlyReachableByFallthrough, updated by
commit 1995b9fe for SPARC, which also
handles MIPS.