rs6000: Generate an lxvp instead of two adjacent lxv instructions

rs6000: Generate an lxvp instead of two adjacent lxv instructions

The MMA build built-ins currently use individual lxv instructions to
load up the registers of a __vector_pair or __vector_quad.  If the
memory addresses of the built-in operands are to adjacent locations,
then we can use an lxvp in some cases to load up two registers at once.
The patch below adds support for checking whether memory addresses are
adjacent and emitting an lxvp instead of two lxv instructions.

2021-07-14  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/rs6000.c (adjacent_mem_locations): Return the lower
	addressed memory rtx, if any.
	(rs6000_split_multireg_move): Fix code formatting.
	Handle MMA build built-ins with operands in adjacent memory locations.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-9.c: New test.