[AArch64] Let LR register alloctable

  gcc/
    * config/aarch64/aarch64.h (CALL_USED_REGISTERS): Mark LR as caller-save.
    (EPILOGUE_USES): Guard the check by epilogue_completed.
    * config/aarch64/aarch64.c (aarch64_layout_frame): Explictly check for LR.
    (aarch64_can_eliminate): Check LR_REGNUM liveness.

  gcc/testsuite/
    * gcc.target/aarch64/lr_free_1.c: New testcase for -fomit-frame-pointer.
    * gcc.target/aarch64/lr_free_2.c: New testcase for leaf
    -fno-omit-frame-pointer.

From-SVN: r217431