Precompute lists of explicit sub-registers and indices.

Precompute lists of explicit sub-registers and indices.

The .td files specify a tree of sub-registers. Store that tree as
ExplicitSubRegs lists in CodeGenRegister instead of extracting it from
the Record when needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@156555 91177308-0d34-0410-b5e6-96231b3b80d8