Add new constant data structure.

Add new constant data structure.

This patch provides the data structure and function to convert a
CONST_INT, CONST_DOUBLE, CONST_VECTOR, or VEC_DUPLICATE of a constant) to
an array of bytes, half-words, words, and  double words that can be loaded
into a 128-bit vector register.

The next patches will use this data structure to generate code that
generates load of the vector/floating point registers using the XXSPLTIDP,
XXSPLTIW, and LXVKQ instructions that were added in power10.

2021-12-15  Michael Meissner  <meissner@the-meissners.org>

gcc/

	* config/rs6000/rs6000-protos.h (VECTOR_128BIT_BITS): New macro.
	(VECTOR_128BIT_BYTES): Likewise.
	(VECTOR_128BIT_HALF_WORDS): Likewise.
	(VECTOR_128BIT_WORDS): Likewise.
	(VECTOR_128BIT_DOUBLE_WORDS): Likewise.
	(vec_const_128bit_type): New structure type.
	(vec_const_128bit_to_bytes): New declaration.
	* config/rs6000/rs6000.c (constant_int_to_128bit_vector): New
	helper function.
	(constant_fp_to_128bit_vector): New helper function.
	(vec_const_128bit_to_bytes): New function.