Revert "TableGen: Switch from a std::map to a DenseMap in CodeGenSubRegIndex. NFC"
Revert "TableGen: Switch from a std::map to a DenseMap in CodeGenSubRegIndex. NFC"

It turns out we iterate over this map a fair amount and the order
matters for clang to be deterministic. See:

  http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20160919/391315.html

This reverts r279875.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@282040 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm