[FastISel] NFC: Clean up unnecessary bookkeeping

Now that we flush the local value map for every instruction, we don't
need any extra flushes for specific cases.  Also, LastFlushPoint is
not used for anything.  Follow-ups to #c161665 (D91734).

This reapplies #3fd39d36.

Differential Revision: https://reviews.llvm.org/D92338