Reapply r238011 with a fix for the trap instruction.

The problem was that I slipped a change required for shrink-wrapping, namely I
used getFirstTerminator instead of the getLastNonDebugInstr that was here before
the refactoring, whereas the surrounding code is not yet patched for that.

Original message:
[X86] Refactor the prologue emission to prepare for shrink-wrapping.

- Add a late pass to expand pseudo instructions (tail call and EH returns).
 Instead of doing it in the prologue emission.
- Factor some static methods in X86FrameLowering to ease code sharing.

NFC.

Related to <rdar://problem/20821487>

llvm-svn: 238035