[AMDGPU] Don't skip regions in getRegionLiveInMap (#151423)

Currently, this skips any region that is not the first region in a
block. This is because the only user of it only cares about the LiveIns
per-block. However, as named, this is supposed to compute the per-region
LiveIns.

This doesn't have any effect on scheduling / CodeGen currently (aside
from computing LiveIns for all regions) since only the per-block LiveIns
are needed. However, I'm working on something that will use this.

Intended User:

#149367


https://github.com/llvm/llvm-project/blob/c62a2f127cba5d6df350474dfd4a6e5f9250fe4f/llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp#L1351

Author: jrbyrnes

llvm/lib/Target/AMDGPU/GCNSchedStrategy.cpp

@@ -896,15 +896,10 @@ GCNScheduleDAGMILive::getRegionLiveInMap() const {
   assert(!Regions.empty());
   std::vector<MachineInstr *> RegionFirstMIs;
   RegionFirstMIs.reserve(Regions.size());
-  auto I = Regions.rbegin(), E = Regions.rend();
-  do {
-    const MachineBasicBlock *MBB = I->first->getParent();
-    auto *MI = &*skipDebugInstructionsForward(I->first, I->second);
-    RegionFirstMIs.push_back(MI);
-    do {
-      ++I;
-    } while (I != E && I->first->getParent() == MBB);
-  } while (I != E);
+  for (auto &[RegionBegin, RegionEnd] : reverse(Regions))
+    RegionFirstMIs.push_back(
+        &*skipDebugInstructionsForward(RegionBegin, RegionEnd));
+
   return getLiveRegMap(RegionFirstMIs, /*After=*/false, *LIS);
 }