[GlobalISel] Add support for address-taken basic blocks

To make this work, pointers from the MachineBasicBlock to the LLVM-IR-level
basic blocks need to be initialized, as the AsmPrinter uses this link to be
able to print out labels for the basic blocks that are address-taken.

Most of the changes in this commit are about adapting existing tests to include
the basic block name that is now printed out in the MIR format, now that the
name becomes available as the link to the LLVM-IR basic block is initialized.
The relevant test change for the functionality added in this patch are the
added "(address-taken)" strings in
test/CodeGen/AArch64/GlobalISel/arm64-irtranslator.ll.

Differential Revision: https://reviews.llvm.org/D28123

llvm-svn: 291105