rs6000: MMA built-in dies with incorrect sharing of tree nodes error

rs6000: MMA built-in dies with incorrect sharing of tree nodes error

When we expand our MMA built-ins into gimple, we erroneously reused the
accumulator memory reference for both the source input value as well as
the destination output value.  This led to a tree sharing error.
The solution is to create separate memory references for the input
and output values.

2020-09-01  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/96808
	* config/rs6000/rs6000-call.c (rs6000_gimple_fold_mma_builtin): Do not
	reuse accumulator memory reference for source and destination accesses.

gcc/testsuite/
	PR target/96808
	* gcc.target/powerpc/pr96808.c: New test.