[llvm] [RegAllocFast] Replace UsedInInstr with vector (PR #96323)

[Matt Arsenault via llvm-commits]

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@@ -253,11 +253,19 @@ class RegAllocFastImpl {
 
   SmallVector<MachineInstr *, 32> Coalesced;
 
-  using RegUnitSet = SparseSet<uint16_t, identity<uint16_t>>;
-  /// Set of register units that are used in the current instruction, and so
+  /// Track register units that are used in the current instruction, and so
   /// cannot be allocated.
-  RegUnitSet UsedInInstr;
-  RegUnitSet PhysRegUses;
+  ///
+  /// In the first phase (tied defs/early clobber), we consider also physical
+  /// uses, afterwards, we don't. if the lowest bit isn't set, it's a solely
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arsenm wrote:

Capitalize if 

https://github.com/llvm/llvm-project/pull/96323