[RegAllocFast] Use uint16_t SparseT for LiveRegMap

[aengelke]

Summary

For functions with very large numbers of live variables, lookups into
LiveRegMap previously detoriated to linear searches.

This slightly increases memory usage, but that is barely measurable.

llvm/lib/CodeGen/RegAllocFast.cpp

struct LiveReg {
 
       explicit LiveReg(Register VirtReg) : VirtReg(VirtReg) {}
 
       unsigned getSparseSetIndex() const {
         return Register::virtReg2Index(VirtReg);
       }
     };
 
-    using LiveRegMap = SparseSet<LiveReg>;
+    using LiveRegMap = SparseSet<LiveReg, identity<unsigned>, uint16_t>;
     /// This map contains entries for each virtual register that is currently
     /// available in a physical register.
     LiveRegMap LiveVirtRegs;
 
     /// Stores assigned virtual registers present in the bundle MI.
     DenseMap<Register, MCPhysReg> BundleVirtRegsMap;
 
     DenseMap<unsigned, SmallVector<MachineOperand *, 2>> LiveDbgValueMap;