[HLSL][SPIRV] Correct reversebit 64 split (#197849)
Replace the split-lanes + OpCompositeConstruct sequence in
selectBitreverse64 with a single OpVectorShuffle to swap the high/low
32-bit halves, simplifying both the selector logic, the emitted SPIR-V
and fixing the bug.
fix: #197810
Assisted by: Claude Opus 4.6
