[GlobalISel] Allow the ArtifactValueFinder to return the best available register on failure.

In some cases, like with inserts, we may have a matching size register already,
but still decide to try to look further. This change adds a CurrentBest
register to the value finder state, and any time a method fails to make progress,
returns that register (which may just be an empty Register).

To facilitate this, add a new entry point to the findValueFromDef() function
which initializes this state.

Also fix the build vector finder to return the current build_vector if all
sources are being requested.

Differential Revision: https://reviews.llvm.org/D107017