forwprop: Change an if into an assert

forwprop: Change an if into an assert

Since the merge of the tuples branch (r0-88576-g726a989a8b74bf), the
if:
```
  if (TREE_CODE_CLASS (gimple_cond_code (stmt)) != tcc_comparison)
```
Will always be false so let's change it into an assert.

gcc/ChangeLog:

	* tree-ssa-forwprop.cc (forward_propagate_into_gimple_cond): Assert
	that gimple_cond_code is always a comparison.

Signed-off-by: Andrew Pinski <quic_apinski@quicinc.com>