[DWARF] Allow duplication of tails with CFI instructions

This commit came as a result for revert of patch r317579 (originally
committed as r317100). The patch made CFI instructions duplicable, because
their existence in the epilogue block was affecting the Tail duplication
pass. However, duplicating blocks with CFI instructions was an issue for
compact unwind info on Darwin, which is why the patch was reverted.

This patch allows duplicating tails with CFI instructions, though they are
not duplicable, by copying them 'manually'.


Patch by Djordje Kovacevic.

Differential Revision: https://reviews.llvm.org/D40979

llvm-svn: 323883