Fix two bugs revealed during internal testing.

Fix two bugs revealed during internal testing.

- gtNewTempAssign was unnecessarily creating a new node to represent the
  value being assigned to a temp if the value was a normalize-on-load
  lclVar. This caused problems when using `gtNewTempAssign` as part of
  `LIR::Use::ReplaceWithLclVar`, which assumes that `gtNewTempAssign`
  only creates a single node (the new `st.lclVar` node). Instead of
  creating a new lclVar node, simply change the type of the existing
  node.
- LowerSignedDivOrMod contained an unnecessarily strong assertion. This
  assertion has been removed.

Commit migrated from https://github.com/dotnet/coreclr/commit/aab69a3bb42398384d60962f331158e1e3e908c1