[AMDGPU][SILowerSGPRSpills] Update the correct pass dependency

Replace the unused analysis (VirtRegMap) dependency with the used one (SlotIndexes).

This initializes `SlotIndexesWrapperPass` which is used by SILowerSGPRSpills to ensure that legacy pass manager finds it.
Removes the initialization for `VirtRegMapWrapperLegacy` pass since it is not requested in this pass.

Author: optimisan

llvm/lib/Target/AMDGPU/SILowerSGPRSpills.cpp

@@ -95,8 +95,8 @@ char SILowerSGPRSpillsLegacy::ID = 0;
 INITIALIZE_PASS_BEGIN(SILowerSGPRSpillsLegacy, DEBUG_TYPE,
                       "SI lower SGPR spill instructions", false, false)
 INITIALIZE_PASS_DEPENDENCY(LiveIntervalsWrapperPass)
-INITIALIZE_PASS_DEPENDENCY(VirtRegMapWrapperLegacy)
 INITIALIZE_PASS_DEPENDENCY(MachineDominatorTreeWrapperPass)
+INITIALIZE_PASS_DEPENDENCY(SlotIndexesWrapperPass)
 INITIALIZE_PASS_END(SILowerSGPRSpillsLegacy, DEBUG_TYPE,
                     "SI lower SGPR spill instructions", false, false)