MC: Simplify fragment reuse determination

First, avoid checking MCSubtargetInfo by reducing unnecessary overhead
introduced in https://reviews.llvm.org/D44928 . That change passed STI
to both FT_Data and FT_Relaxable fragments, but STI is only necessary
for FT_Relaxable.

The use of STI in FT_Data was added for:

* Bundle alignment mode, which has been removed (#148781).
* ARM, which inappropriately uses STI in `ARMAsmBackend::applyFixup` due
  to tech debt, unlike other targets. All tests passed even without the
  `copySTI` change.

To ensure safety, `copySTI` now starts a new fragment to prevent mixed
STI values.

Second, avoid checking LinkerRelaxable by eagerly starting a new
fragment when a FT_Data/FT_Align fragment is marked linker-relaxable.
There is currently an extra empty FT_Data if an alignment immediately
follows a linker-relaxable fragment, which will be improved in the
future when FT_Align information is moved to the variable-tail.

Pull Request: https://github.com/llvm/llvm-project/pull/149471

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