[x86] Fold extract_vector_elt of a load into the Load's address computation.
[MachineSink] Improve the compile time by preserving the dominance information
as long as possible.

** Context **

Each time the dominance information is modified, the dominator tree analysis
switches in a slow query mode. After a few queries without any modification on
the dominator tree, it performs an expensive update of its internal structure to
provide fast queries again.

** Problem **

Prior to this patch, the MachineSink pass was splitting the critical edges on
demand while relying heavy on the dominator tree information. In some cases,
this leads to pathological behavior where:
- We end up in the slow query mode right after splitting an edge.
- We update the dominance information.
- We break the dominance information again, thus ending up in the slow query
  mode and so on.

** Proposed Solution **

To mitigate this effect, this patch postpones all the splitting of the edges at
the end of each iteration of the main loop.
The benefits are:
- The dominance information is valid for the life time of an iteration.
- This simplifies the code as we do not have to special treat instructions that
  are sunk on critical edges. Indeed, the related block will be available
  through the next iteration.

The downside is that when edges splitting is required, this incurs an additional
iteration of the main loop compared to the previous scheme.

** Performance **

Thanks to this patch, the motivating example compiles in 6+ minutes instead of
10+ minutes. No test case added as the motivating example as nothing special but
being huge!

I have measured only noise for both the compile time and the runtime on the llvm
test-suite + SPECs with Os and O3.

Note: The current implementation of MachineBasicBlock::SplitCriticalEdge also
uses the dominance information and therefore, hits this problem. A subsequent
patch will address that.

<rdar://problem/17894619>
Fetching the parent frame may fail, handle that case.  Patch from Tong Shen.