[SelectionDAG] Remove deprecated MemSDNode->getAlignment()

I noticed a an assertion error when building MIPS code that loaded from
NULL. Loading from NULL ends up being a load with maximum alignment, and
due to integer truncation the value maximum was interpreted as 0 and the
assertion in MipsDAGToDAGISel::Select() failed. This previously happened
to work, but the maximum alignment was increased in
df84c1fe, so it no longer fits into a 32
bit integer.
Instead of just fixing the one MIPS case, this patch removes all uses of
the deprecated getAlignment() call and replaces them with getAlign().

Differential Revision: https://reviews.llvm.org/D138420