Fold (add -1; zero_ext; add +1) operations to zero_ext when not overflow(PR37451...

Fold (add -1; zero_ext; add +1) operations to zero_ext when not overflow(PR37451, PR61837)

This "subtract/extend/add" existed for a long time and still annoying us
(PR37451, part of PR61837) when converting from 32bits to 64bits, as the ctr
register is used as 64bits on powerpc64, Andraw Pinski had a patch but
caused some issue and reverted by Joseph S. Myers(PR37451, PR37782).

Andraw:
http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01070.html
http://gcc.gnu.org/ml/gcc-patches/2008-10/msg01321.html
Joseph:
https://gcc.gnu.org/legacy-ml/gcc-patches/2011-11/msg02405.html

We still can do the simplification from "subtract/zero_ext/add" to "zero_ext"
when loop iterations is known to be LT than MODE_MAX (only do simplify
when counter+0x1 NOT overflow).

Bootstrap and regression tested pass on Power8-LE.

gcc/ChangeLog

	2020-05-15  Xiong Hu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/37451, part of PR target/61837
	* loop-doloop.c (doloop_simplify_count): New function.  Simplify
	(add -1; zero_ext; add +1) to zero_ext when not wrapping.
	(doloop_modify): Call doloop_simplify_count.

gcc/testsuite/ChangeLog

	2020-05-15  Xiong Hu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/37451, part of PR target/61837
	* gcc.target/powerpc/doloop-2.c: New test.