[ARM] Refactor the prologue/epilogue emission to be more robust.

This is the first step toward supporting shrink-wrapping for this target.

The changes could be summarized by these items:
- Expand the tail-call return as part of the expand pseudo pass.
- Get rid of the assumptions that the epilogue is the exit block:
  * Do not assume which registers are free in the epilogue. (This indirectly
    improve the lowering of the code for the segmented stacks, see the test
    cases.)
  * Take into account that the basic block can be empty.

Related to <rdar://problem/20821730>

llvm-svn: 242714

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