rs6000: Add support for compatibility built-ins

rs6000: Add support for compatibility built-ins

The LLVM and GCC teams agreed to rename the __builtin_mma_assemble_pair and
__builtin_mma_disassemble_pair built-ins to __builtin_vsx_assemble_pair and
__builtin_vsx_disassemble_pair respectively.  It's too late to remove the
old names, so this patch renames the built-ins to the new names and then
adds support for creating compatibility built-ins (ie, multiple built-in
functions generate the same code) and then creates compatibility built-ins
using the old names.

2021-02-23  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	* config/rs6000/mma.md (mma_assemble_pair): Rename from this...
	(vsx_assemble_pair): ...to this.
	(*mma_assemble_pair): Rename from this...
	(*vsx_assemble_pair): ...to this.
	(mma_disassemble_pair): Rename from this...
	(vsx_disassemble_pair): ...to this.
	(*mma_disassemble_pair): Rename from this...
	(*vsx_disassemble_pair): ...to this.
	* config/rs6000/rs6000-builtin.def (BU_MMA_V2, BU_MMA_V3,
	BU_COMPAT): New macros.
	(mma_assemble_pair): Rename from this...
	(vsx_assemble_pair): ...to this.
	(mma_disassemble_pair): Rename from this...
	(vsx_disassemble_pair): ...to this.
	(mma_assemble_pair): New compatibility built-in.
	(mma_disassemble_pair): Likewise.
	* config/rs6000/rs6000-call.c (struct builtin_compatibility): New.
	(RS6000_BUILTIN_COMPAT): Define.
	(bdesc_compat): New.
	(mma_expand_builtin): Use VSX_BUILTIN_DISASSEMBLE_PAIR_INTERNAL.
	(rs6000_gimple_fold_mma_builtin): Use MMA_BUILTIN_DISASSEMBLE_PAIR
	and VSX_BUILTIN_ASSEMBLE_PAIR.
	(rs6000_init_builtins): Register compatibility built-ins.
	(mma_init_builtins): Use VSX_BUILTIN_ASSEMBLE_PAIR,
	VSX_BUILTIN_ASSEMBLE_PAIR_INTERNAL, VSX_BUILTIN_DISASSEMBLE_PAIR and
	VSX_BUILTIN_DISASSEMBLE_PAIR_INTERNAL.
	* doc/extend.texi (__builtin_mma_assemble_pair): Rename from this...
	(__builtin_vsx_assemble_pair): ...to this.
	(__builtin_mma_disassemble_pair): Rename from this...
	(__builtin_vsx_disassemble_pair): ...to this.

gcc/testsuite/
	* gcc.target/powerpc/mma-builtin-4.c: Add tests for
	__builtin_vsx_assemble_pair and __builtin_vsx_disassemble_pair.
	Add __has_builtin tests for built-ins.
	Update expected instruction counts.