PHIOPT: Mark the conditional lhs and rhs as to look at to see if DCEable

PHIOPT: Mark the conditional lhs and rhs as to look at to see if DCEable

In some cases (usually dealing with bools only), there could be some statements
left behind which are considered trivial dead.
An example is:
```
bool f(bool a, bool b)
{
    if (!a && !b)
        return 0;
    if (!a && b)
        return 0;
    if (a && !b)
        return 0;
    return 1;
}
```
Where during phiopt2, the IR had:
```
  _3 = ~b_7(D);
  _4 = _3 & a_6(D);
  _4 != 0 ? 0 : 1
```
match-and-simplify would transform that into:
```
  _11 = ~a_6(D);
  _12 = b_7(D) | _11;
```
But phiopt would leave around the statements defining _4 and _3.
This helps by marking the conditional's lhs and rhs to see if they are
trivial dead.

OK? Bootstrapped and tested on x86_64-linux-gnu.

gcc/ChangeLog:

	* tree-ssa-phiopt.cc (match_simplify_replacement): Mark's cond
	statement's lhs and rhs to check if trivial dead.
	Rename inserted_exprs to exprs_maybe_dce; also move it so
	bitmap is not allocated if not needed.