[SCEVExpander] Add helper to clean up instrs inserted while expanding.

SCEVExpander already tracks which instructions have been inserted n
InsertedValues/InsertedPostIncValues. This patch adds an additional
vector to collect the instructions in insertion order. This can then be
used to remove exactly the instructions inserted by the expander.

This replaces ExpandedValuesCleaner, which in some cases might remove
values not inserted by the expander (e.g. if a value was dead before
insertion and is then used during expansion).

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D84327