exp/ssa: make invokation of deferred procedure calls explicit.

The correct semantics of named result parameters and deferred
procedures cannot be implemented with the existing Ret
instruction alone, since the required sequence is:
(1) evaluate return operands and parallel-assign them to
    named result parameters
(2) invoke deferred procedures
(3) load named result parameters to form result tuple.

We introduce a new 'rundefers' instruction that explicitly
invokes the deferred procedure calls, and we generate code
that follows the sequence above.

Most functions do not use deferred procedures but this cannot
be known in a single pass.  So, we add an optimisation to
eliminate redundant 'rundefers'; it is piggybacked on the
existing pass done for "lifting".

Added tests.

R=gri
CC=golang-dev
https://codereview.appspot.com/7411043