rs6000: Enable limited unrolling at -O2

rs6000: Enable limited unrolling at -O2

In PR88760, there are a few disscussion about improve or tune unroller for
targets. And we would agree to enable unroller for small loops at O2 first.
And we could see performance improvement(~10%) for below code:
```
  subroutine foo (i, i1, block)
    integer :: i, i1
    integer :: block(9, 9, 9)
    block(i:9,1,i1) = block(i:9,1,i1) - 10
  end subroutine foo

```
This kind of code occurs a few times in exchange2 benchmark.

Similar C code:
```
  for (i = 0; i < n; i++)
    arr[i] = arr[i] - 10;
```

On powerpcle, for O2 , enable -funroll-loops and limit
PARAM_MAX_UNROLL_TIMES=2 and PARAM_MAX_UNROLLED_INSNS=20, we can see >2%
overall improvement for SPEC2017.

This patch is only for rs6000 in which we see visible performance improvement.

gcc/
2019-10-25  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR tree-optimization/88760
	* config/rs6000/rs6000-common.c (rs6000_option_optimization_table):
	Enable -funroll-loops for -O2 and above.
	* config/rs6000/rs6000.c (rs6000_option_override_internal): Set
	PARAM_MAX_UNROLL_TIMES to 2 and PARAM_MAX_UNROLLED_INSNS to 20, and
	do not turn on web and rngreg implicitly, if the unroller is not
	explicitly enabled.

gcc.testsuite/
2019-10-25  Jiufu Guo  <guojiufu@linux.ibm.com>

	PR tree-optimization/88760
	* gcc.target/powerpc/small-loop-unroll.c: New test.
	* c-c++-common/tsan/thread_leak2.c: Update test.
	* gcc.dg/pr59643.c: Update test.
	* gcc.target/powerpc/loop_align.c: Update test.
	* gcc.target/powerpc/ppc-fma-1.c: Update test.
	* gcc.target/powerpc/ppc-fma-2.c: Update test.
	* gcc.target/powerpc/ppc-fma-3.c: Update test.
	* gcc.target/powerpc/ppc-fma-4.c: Update test.
	* gcc.target/powerpc/pr78604.c: Update test.

From-SVN: r277501