Add alternatives for vec_extract with constant element loading from memory.

Add alternatives for vec_extract with constant element loading from memory.

This patch expands the alternatives for doing vec_extract of V4SI, V8HI, and
V16QI vectors with a constant offset when the vector is in memory.  If the
element number is 0 or we are using offsettable addressing for loading up GPR
registers we don't need to allocate a temporary base register.  We can fold the
offset from the vec_extract into the normal address.

I also added alternatives to load the values into vector registers.  If we load
the value into vector registers, we require X-form addressing.

I added the VSX_EX_ISA mode attribute to distinguish that we can load 32-bit
integers on a power8 system to vector registers, but we need a power9 system to
be able to load 8-bit or 16-bit integers.

In general, loading up small integer values with vec_extract into the vector
registers explicitly is likely not done that much.  However, this will be needed
in later patches when we want to combine loading up a small integer value into a
vector register with sign/zero extension.  This happens when we want to do a
vec_extract of a smal integer value and convert it to floating point.

2023-07-11   Michael Meissner  <meissner@linux.ibm.com>

gcc/

	* config/rs6000/vsx.md (VSX_EX_ISA): New mode attribute.
	(vsx_extract_<mode>_load): Add more alternatives for memory options.
	Allow the load to load up vector registers if needed.

gcc/testsuite/

	* gcc.target/powerpc/vec-extract-mem-char-1.c: New test.
	* gcc.target/powerpc/vec-extract-mem-int-1.c: New test.
	* gcc.target/powerpc/vec-extract-mem-short-1.c: New test.