[SelectionDAG] Allow FindMemType to fail when widening loads & stores

This patch removes an internal failure found in FindMemType and "bubbles
it up" to the users of that method: GenWidenVectorLoads and
GenWidenVectorStores. FindMemType -- renamed findMemType -- now returns
an optional value, returning None if no such type is found.

Each of the aforementioned users now pre-calculates the list of types it
will use to widen the memory access. If the type breakdown is not
possible they will signal a failure, at which point the compiler will
crash as it does currently.

This patch is preparing the ground for alternative legalization
strategies for vector loads and stores, such as using vector-predication
versions of loads or stores.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112000