Fold vsx_extract from memory with constant element to floating point.

Fold vsx_extract from memory with constant element to floating point.

This patch folds V4SI/V8HI/V16QI vsx_extract from memory where the element
number is constant and it is converted to floating point (both signed and
unsigned floating point conversion).

2023-04-21   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/vsx.md (FL_CONV_REG): New mode attribute.
	(vsx_extract_<VSX_EXTRACT_I:mode>_load_to_u<FL_CONV:mode>): New insn.
	(vsx_extract_v4si_load_to_s<mode>): New insn.
	(vsx_extract_v8hi_load_to_s<mode>): New insn.