rs6000: Don't ICE when spilling an MMA accumulator

rs6000: Don't ICE when spilling an MMA accumulator

When we spill an accumulator that has a known zero value, LRA will emit
a new (set (reg:PXI ...) 0) insn, but it does not use the mma_xxsetaccz
pattern to do it, leading to an unrecognized insn ICE.  The solution here
is to move the xxsetaccz instruction into the movpxi pattern and have the
xxsetaccz pattern call the move pattern.

2020-08-06  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/96446
	* config/rs6000/mma.md (*movpxi): Add xxsetaccz generation.
	Disable split for zero constant source operand.
	(mma_xxsetaccz): Change to define_expand.  Call gen_movpxi.

gcc/testsuite/
	PR target/96446
	* gcc.target/powerpc/pr96446.c: New test.