[MachineSink] Use SmallDenseMap (NFC) (#95676)

The use of SmallDenseMap saves 0.39% of heap allocations during the
compilation of a large preprocessed file, namely X86ISelLowering.cpp,
for the X86 target.

Author: kazutakahirata

llvm/lib/CodeGen/MachineSink.cpp

@@ -138,7 +138,7 @@ namespace {
     DenseSet<Register> RegsToClearKillFlags;
 
     using AllSuccsCache =
-        DenseMap<MachineBasicBlock *, SmallVector<MachineBasicBlock *, 4>>;
+        SmallDenseMap<MachineBasicBlock *, SmallVector<MachineBasicBlock *, 4>>;
 
     /// DBG_VALUE pointer and flag. The flag is true if this DBG_VALUE is
     /// post-dominated by another DBG_VALUE of the same variable location.