Support for testing GIMPLE passes

As of gcc 7, C functions can be tagged with __GIMPLE to indicate that the function body will be GIMPLE, rather than C. The compiler requires the option -fgimple to enable this functionality. For example:

/* { dg-do compile } */
/* { dg-options "-O -fgimple" } */

void __GIMPLE (startwith ("dse2")) foo ()
{
  int a;

bb_2:
  if (a > 4)
    goto bb_3;
  else
    goto bb_4;

bb_3:
  a_2 = 10;
  goto bb_5;

bb_4:
  a_3 = 20;

bb_5:
  a_1 = __PHI (bb_3: a_2, bb_4: a_3);
  a_4 = a_1 + 4;

  return;
}

The startwith argument indicates at which pass to begin.

Use the dump modifier -gimple (e.g. -fdump-tree-all-gimple) to make tree dumps more closely follow the format accepted by the GIMPLE parser.

Example DejaGnu tests of GIMPLE can be seen in the source tree at gcc/testsuite/gcc.dg/gimplefe-*.c.

The __GIMPLE parser is integrated with the C tokenizer and preprocessor, so it should be possible to use macros to build out test coverage.