bpf: avoid load from read-only sections

If users tried to have a structure decl/init code like below
   struct test_t t = { .memeber1 = 45 };
It is very likely that compiler will generate a readonly section
to hold up the init values for variable t. Later load of t members,
e.g., t.member1 will result in a read from readonly section.

BPF program cannot handle relocation. This will force users to
write:
  struct test_t t = {};
  t.member1 = 45;
This is just inconvenient and unintuitive.

This patch addresses this issue by implementing BPF PreprocessISelDAG.
For any load from a global constant structure or an global array of
constant struct, it attempts to
translate it into a constant directly. The traversal of the
constant struct and other constant data structures are similar
to where the assembler emits read-only sections.

Four different unit test cases are also added to cover
different scenarios.

Signed-off-by: Yonghong Song <yhs@fb.com>
llvm-svn: 305560