Calling ReassociateExpression recursively is extremely dangerous since it will

replace the operands of expressions with only one use with undef and generate
a new expression for the original without using RAUW to update the original.
Thus any copies of the original expression held in a vector may end up
referring to some bogus value - and using a ValueHandle won't help since there
is no RAUW.  There is already a mechanism for getting the effect of recursion
non-recursively: adding the value to be recursed on to RedoInsts.  But it wasn't
being used systematically.  Have various places where recursion had snuck in at
some point use the RedoInsts mechanism instead.  Fixes PR12169.

llvm-svn: 156379