middle-end: Fix ifcvt predicate generation for masked function calls

Up until now, due to a latent bug in the code for the ifcvt pass,
irrespective of the branch taken in a conditional statement, the
original condition for the if statement was used in masking the
function call.

Thus, for code such as:

  if (a[i] > limit)
    b[i] = fixed_const;
  else
    b[i] = fn (a[i]);

we would generate the following (wrong) if-converted tree code:

  _1 = a[i_1];
  _2 = _1 > limit;
  _3 = .MASK_CALL (fn, _1, _2);
  cstore_4 = _2 ? fixed_const : _3;

as opposed to the correct expected sequence:

  _1 = a[i_1];
  _2 = _1 > limit;
  _3 = ~_2;
  _4 = .MASK_CALL (fn, _1, _3);
  cstore_5 = _2 ? fixed_const : _4;

This patch ensures that the correct predicate mask generation is
carried out such that, upon autovectorization, the correct vector
lanes are selected in the vectorized function call.

gcc/ChangeLog:

	* tree-if-conv.cc (predicate_statements): Fix handling of
	predicated function calls.

gcc/testsuite/ChangeLog:

	* gcc.dg/vect/vect-fncall-mask.c: New.