Allow vsx_extract_<mode> to use Altivec registers.

I noticed that the vsx_extract_<mode> pattern for V2DImode and V2DFmode
only allowed traditional floating point registers, and it did not allow
Altivec registers.  The original code was written a few years ago when we
used the old register allocator, and support for scalar floating point in
Altivec registers was just being added to GCC.

I have built the spec 2017 benchmark suite With all 4 patches in this
series applied, and compared it to the build with the previous 3 patches
applied.  In addition to the changes from the previous 3 patches, this
patch now changes the code for the following 3 benchmarks (2 floating
point, 1 integer):

	bwaves_r, fotonik3d_r, xalancbmk_r

I have built bootstrap versions on the following systems.  There were no
regressions in the runs:

	Power9 little endian, --with-cpu=power9
	Power10 little endian, --with-cpu=power10
	Power8 big endian, --with-cpu=power8 (both 32-bit & 64-bit tests)

2022-03-29   Michael Meissner  <meissner@linux.ibm.com>

gcc/
	* config/rs6000/vsx.md (vsx_extract_<mode>): Allow destination to
	be any VSX register.