[Constant Hoisting] Don't update the use list while traversing it - DOH!

This fix first traverses the whole use list of the constant expression and
keeps track of the instructions that need to be updated. Then perform the
fixup afterwards.
[Constant Hoisting] Fix insertion point for constant materialization.

The bitcast instruction during constant materialization was not placed correcly
in the presence of phi nodes. This commit fixes the insertion point to be in the
idom instead.

This fixes PR18768