rs6000: Fix ICE expanding lxvp and stxvp gimple built-ins [PR101849]

rs6000: Fix ICE expanding lxvp and stxvp gimple built-ins [PR101849]

PR101849 shows we ICE on a test case when we pass a non __vector_pair *
pointer to the __builtin_vsx_lxvp and __builtin_vsx_stxvp built-ins
that is cast to __vector_pair *.  The problem is that when we expand
the built-in, the cast has already been removed from gimple and we are
only given the base pointer.  The solution used here (which fixes the ICE)
is to catch this case and convert the pointer to a __vector_pair * pointer
when expanding the built-in.

2021-08-19  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/101849
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_mma_builtin): Cast
	pointer to __vector_pair *.

gcc/testsuite/
	PR target/101849
	* gcc.target/powerpc/pr101849.c: New test.