[SelectionDAG][NFCI] Use common logic for identifying MMI vars

After function argument lowering, but prior to instruction selection,
dbg declares pointing to function arguments are lowered using special
logic.

Later, during instruction selection (both "fast" and regular ISel), this
logic is "repeated" in order to identify which intrinsics have already
been lowered. This is bad for two reasons:

1. The logic is not _really_ repeated, the code is different, which
could lead to duplicate lowering of the intrinsic.
2. Even if the logic were repeated properly, this is still code
duplication.

This patch addresses these issues by storing all preprocessed
dbg.declare intrinsics in a set inside FuncInfo; the set is queried upon
instruction selection.

Differential Revision: https://reviews.llvm.org/D149682