Store CodeGenRegisters as pointers so they won't be reallocated.

Store CodeGenRegisters as pointers so they won't be reallocated.

Reuse the CodeGenRegBank DenseMap in a few places that would build their
own or use linear search.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@133333 91177308-0d34-0410-b5e6-96231b3b80d8