Don't document exactly how virtual registers are represented as integers. Code

Don't document exactly how virtual registers are represented as integers. Code
shouldn't depend directly on that.

Give an example of how to iterate over all virtual registers in a function
without depending on the representation.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123099 91177308-0d34-0410-b5e6-96231b3b80d8