Failures in store.exp caused by regcache

[Daniel Jacobowitz <drow@mvista.com>]

(Or rather, by the value code's interaction with the regcache)

Andrew, this is more your area; I'd like your advice before I dig any
further.  Here's what's going wrong.  Consider the command sequence:
"up; print u; set u = s_1; print u".
  - u has class LOC_REGISTER
  - The register's home is memory
  - read_var_value therefore returns an lval_memory
  - the value of the register is in the register unwind cache at this point
  - we modify the memory backing the store
  - we have no way to tell that we've just modified the value of a saved
    register on the stack
  - the second print returns the cached value

So, what do we do?  I really don't want to flush cached register values in
value_assign.

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer