Avoid adding loop-carried ops to long chains

Avoid adding loop-carried ops to long chains

Avoid adding loop-carried ops to long chains, otherwise the whole chain will
have dependencies across the loop iteration. Just keep loop-carried ops in a
separate chain.
   E.g.
   x_1 = phi(x_0, x_2)
   y_1 = phi(y_0, y_2)

   a + b + c + d + e + x1 + y1

   SSA1 = a + b;
   SSA2 = c + d;
   SSA3 = SSA1 + e;
   SSA4 = SSA3 + SSA2;
   SSA5 = x1 + y1;
   SSA6 = SSA4 + SSA5;

With the patch applied, these test cases improved by 32%~100%.

S242:
for (int i = 1; i < LEN_1D; ++i) {
    a[i] = a[i - 1] + s1 + s2 + b[i] + c[i] + d[i];}

Case 1:
for (int i = 1; i < LEN_1D; ++i) {
    a[i] = a[i - 1] + s1 + s2 + b[i] + c[i] + d[i] + e[i];}

Case 2:
for (int i = 1; i < LEN_1D; ++i) {
    a[i] = a[i - 1] + b[i - 1] + s1 + s2 + b[i] + c[i] + d[i] + e[i];}

The value is the execution time
A: original version
B: with FMA patch g:e5405f065bace0685cb3b8878d1dfc7a6e7ef409(base on A)
C: with current patch(base on B)

	  A 	  B 	  C 	B/A	        C/A
s242	2.859	5.152	2.859	1.802028681	1
case 1	5.489	5.488	3.511	0.999818	0.64
case 2	7.216	7.499	4.885	1.039218	0.68

gcc/ChangeLog:

	PR tree-optimization/110148
	* tree-ssa-reassoc.cc (rewrite_expr_tree_parallel): Handle loop-carried
	ops in this function.