Revert "Revert "Mark vastart_save_xmm_regs as changing EFLAGS""

This reverts commit r197481, recommiting r197469 with an extra fix.

The vastart_save_xmm_regs pseudo-instruction expands to a test and a
branch, so it modifies EFLAGS.  Mark it so, or else the scheduler might
place it in the middle of another test+branch.

This fixes a bug exposed by r192750, which changed the initial scheduler
to source-order as part of enabling the MI Scheduler for X86.

This re-commit changes the VASTART_SAVE_XMM_REGS custom inserter not to
try to save %flags, and adds a test that catches the bad behavior of
r197469.

<rdar://problem/15627766>