[Hexagon] Add coverage tests for CodeGen passes (#183951)
Add tests targeting specific Hexagon CodeGen passes with low coverage:
- peephole-sxtw-combine.mir: HexagonPeephole pass exercising SXTW
removal, combine generation, and LSR copy patterns. Improves
HexagonPeephole.cpp line coverage from 63.89% to 99.31%.
- vec-print-wq.ll: HexagonVectorPrint pass with V (single vector) and W
(double vector) register printing via 128b HVX. Improves
HexagonVectorPrint.cpp line coverage from 71.19% to 87.29%.
- tfr-cleanup-double-imm.mir: HexagonTfrCleanup pass exercising 64-bit
immediate rewrite paths. Improves HexagonTfrCleanup.cpp line coverage
from 80.85% to 88.30%.
- cfgopt-newpt-invert.ll: HexagonCFGOptimizer pass exercising branch
inversion with new-value predicate transfers.
