rs6000: Disassemble opaque modes using subregs to allow optimizations

rs6000: Disassemble opaque modes using subregs to allow optimizations

PR109116  reveals  missed optimizations  when using  unspecs to  extract
vector  components  from opaque-mode  variables. Since RTL optimizers do
not understand unspecs, this leads to redundant register copies. Replace
unspecs with subregs, which  are well understood by RTL passes, allowing
optimizations to take place.

2025-06-30  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/109116
	* config/rs6000/mma.md (unspec): Delete UNSPEC_MMA_EXTRACT.
	(vsx_disassemble_pair): Expand into a vector register sized subreg.
	(mma_disassemble_acc): Likewise.
	(*vsx_disassemble_pair): Delete.
	(*mma_disassemble_acc): Likewise.