[Mips][AsmPrinter] Fix for assembler not printing a label

[urosbericsyrmia]

This PR is a fix for a bug that manifests in rust-lang/rust#108835.
rustc-demangle couldn't compile due to an undefined temporary symbol error which arises when the assembly code is branching to a basic block whose label is not emitted.
One of the reasons not to emit the label of a basic block is when that block is only reachable by fallthrough. If a block has only one predecessor and that predecessor is immediately before it, the block is reachable only by fallthrough. The problem is that the MipsAsmPrinter didn't take it into account what if that predecessor also branches to this basic block, which in this case happens. In that case, we need a label for the block.
The solution is to check if the predecessor branches to this basic block.

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[MaskRay]

Thanks for the analysis. I think we can just remove the derived isBlockOnlyReachableByFallthrough #80799

[urosbericsyrmia]

Thanks for the feedback. Should I close this one?

[MaskRay]

Thanks for the feedback. Should I close this one?

Sounds good since Fast-ISel/pr40325.ll covers the case.