rs6000: Skip unnecessary vector extract for certain elements.

rs6000: Skip unnecessary vector extract for certain elements.

If the extracted element index is:
  - for byte, 7 on BE while 8 on LE;
  - for half word, 3 on BE while 4 on LE;

the element to be stored is already in the corresponding place for
stxsi[hb]x. We don't need a redundant vector extraction at all.

gcc/
	PR target/110429
	* config/rs6000/vsx.md (*vsx_extract_<mode>_store_p9): Skip vector
	extract when the element is 7 on BE while 8 on LE for byte or 3 on
	BE while 4 on LE for halfword.

gcc/testsuite/
	PR target/110429
	* gcc.target/powerpc/pr110429.c: New.