[BOLT] Fix iterator bugs (#190978)
Fix iterator misuse in four BOLT passes, caught by _GLIBCXX_DEBUG
(enabled via LLVM_ENABLE_EXPENSIVE_CHECKS=ON).
* AllocCombiner: combineAdjustments() erases instructions while
iterating in reverse via llvm::reverse(BB), invalidating the reverse
iterator. Defer erasures to after the loop using a SmallVector.
* ShrinkWrapping: processDeletions() uses
std::prev(BB.eraseInstruction(II)) which is undefined when II ==
begin(). Restructure to standard forward iteration with erase.
* DataflowAnalysis: run() unconditionally dereferences BB->rbegin(),
which crashes on empty basic blocks (possible after the ShrinkWrapping
fix). Guard with an emptiness check.
* IndirectCallPromotion: rewriteCall() dereferences the end iterator via
&(*IndCallBlock.end()). Replace with &IndCallBlock.back().
* TailDuplication: constantAndCopyPropagate() uses
std::prev(OriginalBB.eraseInstruction(Itr)) which is undefined when Itr
== begin(). Restructure to standard forward iteration with erase.
