cloog_loop_merge: optionally compute simplified convex hulls.

cloog_loop_merge: optionally compute simplified convex hulls.

This patch add a "sh" (simple hull) option that can simplify
some constraints at the expense of evaluating the constraints
more often.
Note that this simplification only applies to code generated
without code duplication.

For example,

bash-3.00$ cat union.cloog
c

1
0 2

0

2
1
4 4
1 1 0 0
1 -1 0 10
1 0 1 0
1 0 -1 10

0 0 0

1
4 4
1 1 0 -1
1 -1 0 11
1 0 1 -5
1 0 -1 15

0 0 0

0
0
0
bash-3.00$ ../obj_i686-pc-linux-gnu/cloog -f -1 -l 2 -override union.cloog
[CLooG]WARNING: you are using -override option, be aware that the generated
                code may be incorrect.
/* Generated from union.cloog by CLooG v0.14.0 gmp bits in 0.00s. */
/* CLooG asked for 68 KBytes. */
[CLooG]INFO: 0.00s and 68KB used for code generation.
for (i=0;i<=11;i++) {
  for (j=max(0,5*i-50);j<=min(15,5*i+10);j++) {
    if ((i <= 10) && (j <= 10)) {
      S1 ;
    }
    if ((i >= 1) && (j >= 5)) {
      S2 ;
    }
  }
}
bash-3.00$ ../obj_i686-pc-linux-gnu/cloog -sh 1 -f -1 -l 2 -override union.cloog
[CLooG]WARNING: you are using -override option, be aware that the generated
                code may be incorrect.
/* Generated from union.cloog by CLooG v0.14.0 gmp bits in 0.00s. */
/* CLooG asked for 76 KBytes. */
[CLooG]INFO: 0.00s and 76KB used for code generation.
for (i=0;i<=11;i++) {
  for (j=0;j<=15;j++) {
    if ((i <= 10) && (j <= 10)) {
      S1 ;
    }
    if ((i >= 1) && (j >= 5)) {
      S2 ;
    }
  }
}