[SelectionDAG] Fix usage of Align constructing MachineMemOperands.

The "Align" passed into getMachineMemOperand etc. is the alignment of
the MachinePointerInfo, not the alignment of the memory operation.
(getAlign() on a MachineMemOperand automatically reduces the alignment
to account for this.)

We were passing on wrong (overconservative) alignment in a bunch of
places. Fix a bunch of these, mostly in legalization.  And while I'm
here, switch to the new Align APIs.

The test changes are all scheduling changes: the biggest effect of
preserving large alignments is that it improves alias analysis, so the
scheduler has more freedom.

(I was originally just trying to do a minor cleanup in
SelectionDAGBuilder, but I accidentally went deeper down the rabbit
hole.)

Differential Revision: https://reviews.llvm.org/D77687