bpf: fix a strict-aliasing issue

Davide Italiano reported the following issue if llvm
is compiled with gcc -Wstrict-aliasing -Werror:
.....
lib/Target/BPF/CMakeFiles/LLVMBPFCodeGen.dir/BPFISelDAGToDAG.cpp.o
../lib/Target/BPF/BPFISelDAGToDAG.cpp: In member function ‘virtual
void {anonymous}::BPFDAGToDAGISel::PreprocessISelDAG()’:
../lib/Target/BPF/BPFISelDAGToDAG.cpp:264:26: warning: dereferencing
type-punned pointer will break strict-aliasing rules
[-Wstrict-aliasing]
       val = *(uint16_t *)new_val;
.....

The error is caused by my previous commit (revision 305560).

This patch fixed the issue by introducing an union to avoid
type casting.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 305608