rs6000: unnecessary clear after vctzlsbb in vec_first_match_or_eos_index

For rs6000 target we dont need zero_extend after vctzlsbb as vctzlsbb
already zero extend.

2023-09-17  Ajit Kumar Agarwal  <aagarwa1@linux.ibm.com>

gcc/ChangeLog:

	* config/rs6000/vsx.md (*vctzlsbb_zext_<mode>): New define_insn.

gcc/testsuite/ChangeLog:

	* g++.target/powerpc/altivec-19.C: New testcase.