Propagate bug fix into new code generator

[simonpj]

We must not forget to propagate this fix into the new code generator pipeline:

Thu Mar 25 04:03:28 PDT 2010  Simon Marlow <marlowsd@gmail.com>
  * do_checks: do not set HpAlloc if the stack check fails
  
  This fixes a very rare heap corruption bug, whereby
  
   - a context switch is requested, which sets HpLim to zero
     (contextSwitchCapability(), called by the timer signal or
     another Capability).
  
   - simultaneously a stack check fails, in a code fragment that has
     both a stack and a heap check.
  
  The RTS then assumes that a heap-check failure has occurred and
  subtracts HpAlloc from Hp, although in fact it was a stack-check
  failure and retreating Hp will overwrite valid heap objects.  The bug
  is that HpAlloc should only be set when Hp has been incremented by the
  heap check.  See comments in rts/HeapStackCheck.cmm for more details.
  
  This bug is probably incredibly rare in practice, but I happened to be
  working on a test that triggers it reliably:
  concurrent/should_run/throwto001, compiled with -O -threaded, args 30
  300 +RTS -N2, run repeatedly in a loop.

    M ./compiler/codeGen/CgHeapery.lhs -6 +16

[simonmar]

The new codegen does not have this bug.