VirtRegMap: Correctly deal with bundles when deleting identity copies.

This fixes two problems when VirtRegMap encounters bundles:

- When substituting a vreg subregister def with an actual register the
  internal read flag must be cleared.
- Removing an identity COPY from a bundle needs to use
  removeFromBundle() and a newly introduced function to update
  SlotIndexes.

No testcase here, because none of the in-tree targets trigger this,
however an upcoming commit of mine will need this and the testcase there
will trigger this.

Differential Revision: https://reviews.llvm.org/D30925

llvm-svn: 298024