[RegAllocFast] Use uint16_t SparseT for LiveRegMap

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.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D149330

llvm/lib/CodeGen/RegAllocFast.cpp

@@ -106,7 +106,7 @@ namespace {
       }
     };
 
-    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;