Use the MachineBasicBlock symbol for a callbr target

Summary:
Inline asm doesn't use labels when compiled as an object file. Therefore, we
shouldn't create one for the (potential) callbr destination. Instead, use the
symbol for the MachineBasicBlock.

Reviewers: nickdesaulniers, craig.topper

Reviewed By: nickdesaulniers

Subscribers: xbolva00, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64888

llvm-svn: 366523