Power10: Add BRD, BRW, and BRH support.

Power10: Add BRD, BRW, and BRH support.

This patch adds support for the ISA 3.1 (power10) instructions that does a byte
swap of values in GPR registers.

gcc/
2020-08-07  Michael Meissner  <meissner@linux.ibm.com>

	* config/rs6000/rs6000.md (bswaphi2_reg): Add ISA 3.1 support.
	(bswapsi2_reg): Add ISA 3.1 support.
	(bswapdi2): Rename bswapdi2_xxbrd to bswapdi2_brd.
	(bswapdi2_brd,bswapdi2_xxbrd): Rename.  Add ISA 3.1 support.

gcc/testsuite/
2020-08-07  Michael Meissner  <meissner@linux.ibm.com>

	* gcc.target/powerpc/bswap-brd.c: New test.
	* gcc.target/powerpc/bswap-brw.c: New test.
	* gcc.target/powerpc/bswap-brh.c: New test.