Inline: Fix single-block loop caller cases

Inline: Fix single-block loop caller cases

If the caller block is a single-block loop and inlining will
replace the caller block by several blocks, then:
- The original OpLoopMerge instruction will end up in the *last*
  such block.  That's the wrong place to put it.
- Move it back to the end of the first block.
- Update its Continue Target ID to point to the last block

We also have to take care of cases where the inlined code
begins with a structured header block.  In this case
we need to ensure the restored OpLoopMerge does not appear
in the same block as the merge instruction from the callee's
first block.

Fixes https://github.com/KhronosGroup/SPIRV-Tools/issues/787