[PATCH, PR target/94954] Fix wrong codegen for vec_pack_to_short_fp32() builtin

Hi,
  Fix codegen for builtin vec_pack_to_short_fp32.  This includes adding
  a define_insn for xvcvsphp, and adding a new define_expand for
  convert_4f32_8f16.

[v2]
  Comment on altivec.md "convert_4f32_8f16" enhanced.
  Testsuite builtins-1-p9-runnable.c updated.

  OK for trunk and backports?

  Thanks
  -Will

PR target/94954

gcc/Changelog:

	* config/rs6000/altivec.h (vec_pack_to_short_fp32): Update.
	* config/rs6000/altivec.md (UNSPEC_CONVERT_4F32_8F16): New unspec.
	(convert_4f32_8f16): New define_expand
	* config/rs6000/rs6000-builtin.def (convert_4f32_8f16): New builtin define
	and overload.
	* config/rs6000/rs6000-call.c (P9V_BUILTIN_VEC_CONVERT_4F32_8F16): New
	overloaded builtin entry.
	* config/rs6000/vsx.md (UNSPEC_VSX_XVCVSPHP): New unspec.
	(vsx_xvcvsphp): New define_insn.

gcc/testsuite/ChangeLog:
	* gcc.target/powerpc/builtins-1-p9-runnable.c: Update.