rs6000: Eliminate zext fed by vclzlsbb [PR111480]

As PR111480 shows, commit r14-4079 only optimizes the case
of vctzlsbb but not for the similar vclzlsbb.  This patch
is to consider vclzlsbb as well and avoid the failure on
the reported test case.  It also simplifies the patterns
with iterator and attribute.

	PR target/111480

gcc/ChangeLog:

	* config/rs6000/vsx.md (VCZLSBB): New int iterator.
	(vczlsbb_char): New int attribute.
	(vclzlsbb_<mode>, vctzlsbb_<mode>): Merge to ...
	(vc<vczlsbb_char>zlsbb_<mode>): ... this.
	(*vctzlsbb_zext_<mode>): Rename to ...
	(*vc<vczlsbb_char>zlsbb_zext_<mode>): ... this, and extend it to
	cover vclzlsbb.