i965/fs: Don't use if_block/else_block in SEL peephole.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
i965/cfg: Remove if_block/else_block.

I used these in the SEL peephole, but they require extra tracking and
fix ups. The SEL peephole can pretty easily find the blocks it needs
without these.

Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
gallivm: Fix build with LLVM 3.6 (r221751).

Tested with LLVM 3.3, 3.4, 3.5, and 3.6.

Trivial.