BOLT: Remove double jumps peephole.

Summary:
Replace jumps to other unconditional jumps with the final
destination, e.g.

  B0: ...
      jmp B1  (or jcc B1)

  B1: jmp B2

  ->

  B0: ...
      jmp B2  (or jcc B1)

This peephole removes 8928 double jumps from a test binary.

Note: after filtering out double jumps found in EH code and infinite
loops, the number of double jumps patched is 49 (24 for a clang
compiled test).  The 24 in the clang build are all from external
libraries which have probably been compiled with gcc.  This peephole
is still useful for cleaning up after ICP though.

(cherry picked from FBD3815420)