Re-apply "SelectionDAG: Store SDNode operands in an ArrayRecycler"

This re-applies r262886 with a fix for 32 bit platforms that have 8 byte
pointer alignment, effectively reverting r262892.

Original Message:

  Currently some SDNode operands are malloc'd, some are stored inline in
  subclasses of SDNode, and some are thrown into a BumpPtrAllocator.
  This scheme is complex, inconsistent, and makes refactoring SDNodes
  fairly difficult.

  Instead, we can allocate all of the operands using an ArrayRecycler
  that wraps a BumpPtrAllocator. This keeps the cache locality when
  iterating operands, improves locality when iterating SDNodes without
  looking at operands, and vastly simplifies the ownership semantics.

  It also means we stop overallocating SDNodes by 2-3x and will make it
  simpler to fix the rampant undefined behaviour we have in how we
  mutate SDNodes from one kind to another (See llvm.org/pr26808).

  This is NFC other than the changes in memory behaviour, and I ran some
  LNT tests to make sure this didn't hurt compile time. Not many tests
  changed: there were a couple of 1-2% regressions reported, but there
  were more improvements (of up to 4%) than regressions.

llvm-svn: 262902