rs6000: Enable const_anchor for 'addi'

There is a functionality as const_anchor in cse.cc.  This const_anchor
supports to generate new constants through adding small gap/offsets to
existing constant.  For example:

void __attribute__ ((noinline)) foo (long long *a)
{
  *a++ = 0x2351847027482577LL;
  *a++ = 0x2351847027482578LL;
}
The second constant (0x2351847027482578LL) can be compated by adding '1'
to the first constant (0x2351847027482577LL).
This is profitable if more than one instructions are need to build the
second constant.

* For rs6000, we can enable this functionality, as the instruction
'addi' is just for this when gap is smaller than 0x8000.

* One potential side effect of this feature:
Comparing with
"r101=0x2351847027482577LL
...
r201=0x2351847027482578LL"
The new r201 will be "r201=r101+1", and then r101 will live longer,
and would increase pressure when allocating registers.
But I feel, this would be acceptable for this const_anchor feature.

With this feature, for GCC source code and SPEC object files, the
significant changes are the improvement that: "addi" vs. "2 or more
insns: lis+or.."; it also exposes some other optimizations
opportunities: like combine/jump2. While the side effect is also
occurring in few cases, but it does not impact overall performance.

gcc/ChangeLog:

	* config/rs6000/rs6000.cc (TARGET_CONST_ANCHOR): New define.

gcc/testsuite/ChangeLog:

	* gcc.target/powerpc/const_anchors.c: New test.
	* gcc.target/powerpc/try_const_anchors_ice.c: New test.

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