Internal value caching doesn't track RAUW and deletion

[smaillet]

Found while testing the full integration and support of OrcJit:

Repro:
Run Kaleidoscope 6 or 7 with the sample Kaleidoscope code from the LLVM Tutorial.

# Logical unary not.
def unary!(v)
  if v then
    0
  else
    1;

# Unary negate.
def unary-(v)
  0-v;

# Define > with the same precedence as <.
def binary> 10 (LHS RHS)
  RHS < LHS;

# Binary logical or, which does not short circuit.
def binary| 5 (LHS RHS)
  if LHS then
    1
  else if RHS then
    1
  else
    0;

# Binary logical and, which does not short circuit.
def binary& 6 (LHS RHS)
  if !LHS then
    0
  else
    !!RHS;

# Define = with slightly lower precedence than relationals.
def binary = 9 (LHS RHS)
  !(LHS < RHS | LHS > RHS);

# Define ':' for sequencing: as a low-precedence operator that ignores operands
# and just returns the RHS.
def binary : 1 (x y) y;

Root Cause:
The internal value caching doesn't track Replace All Uses With and deletion operations for Values. If the context is shared by a number of modules that come and go or the functions within them are transformed, then it is highly likely that any number of them will become invalid because some transform replaced or removed an instruction etc.. This ends up with a dangling pointer from managed code side. This becomes a blatant fail when LLVM has re-used the memory for some other type, which may not be a Value at all causing a crash, or if it is a value an InvalidCastException because the resolving to a specific wrapped type doesn't match the new reality.