Revert r263460: [SpillPlacement] Fix a quadratic behavior in spill placement.

Revert r263460: [SpillPlacement] Fix a quadratic behavior in spill placement.

That commit looks wonderful and awesome. Sadly, it greatly exacerbates
PR17409 and effectively regresses build time for a lot of (very large)
code when compiled with ASan or MSan.

We thought this could be fixed forward by landing D15302 which at last
fixes that PR, but some issues were discovered and it looks like that
got reverted, so reverting this as well temporarily. As soon as the fix
for PR17409 lands and sticks, we should re-land this patch as it won't
trigger more significant test cases hitting that bug.

Many thanks to Quentin and Wei here as they're doing all the awesome
hard work!!!

llvm-svn: 265331