rs6000: Fix loop limit for built-in constant checking

The loop checking for built-in constant operand restrictions was missing
some operands due to the loop limit being too small.  Fixing that exposed
a testsuite failure which is caused by a typo in the pmxvi4ger8pp definition
where we had made the PMASK field too small.

2025-01-16  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000-builtin.cc (rs6000_expand_builtin): Use correct
	array size for the loop limit.
	* config/rs6000/rs6000-builtins.def: Fix field size for PMASK operand.