[RegAlloc] Add scaffold for -regalloc=segmenttree (NFC)

[steven-studio]

Adds a minimal factory + regalloc registration for . Currently delegates to Greedy allocator. No functional change.This patch introduces a scaffold for a new register allocator option
-regalloc=segmenttree, which currently delegates to the Greedy allocator.
No functional change is intended.

Motivation:

• Provide a separate regalloc entry point for future work on a
  segment-tree-based allocator.
• Enable early benchmarking and integration testing under a new mode.

Implementation:

• Added RegAllocSegmentTree.cpp / .h containing a factory function
  createRegAllocSegmentTree().
• Registered this allocator in RegisterRegAlloc so that
  llc -regalloc=segmenttree is recognized.
• Hooked up pass initialization in CodeGen.cpp.

Testing:

• Verified that llc -regalloc=segmenttree produces identical codegen
  to -regalloc=greedy.

Future work:

• This is step 1 of a planned series to implement a segment-tree-based
  register allocator, targeting O(V · R · A · S · log E) allocation time
  complexity.

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