[ARM] Fix compiler warning

Without this fix clang 3.6 complains with:

../lib/Target/ARM/ARMAsmPrinter.cpp:1473:18: error: variable 'BranchTarget' is used uninitialized whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
      } else if (MI->getOperand(1).isSymbol()) {
                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~
../lib/Target/ARM/ARMAsmPrinter.cpp:1479:22: note: uninitialized use occurs here
      MCInst.addExpr(BranchTarget);
                     ^~~~~~~~~~~~
../lib/Target/ARM/ARMAsmPrinter.cpp:1473:14: note: remove the 'if' if its condition is always true
      } else if (MI->getOperand(1).isSymbol()) {
             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../lib/Target/ARM/ARMAsmPrinter.cpp:1465:33: note: initialize the variable 'BranchTarget' to silence this warning
      const MCExpr *BranchTarget;
                                ^
                                 = nullptr
1 error generated.

Discussed here:
 http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190610/661417.html

llvm-svn: 363166