nvptx: support bar.red instruction

This patch adds support for the PTX 'bar.red' (i.e. "barrier reduction")
instruction, in the form of nvptx-specific __builtin_nvptx_bar_red_[and/or/popc]
built-in functions.

gcc/ChangeLog:

	* config/nvptx/nvptx.cc (nvptx_print_operand): Add 'p' case, adjust
	comments.
	(enum nvptx_builtins): Add NVPTX_BUILTIN_BAR_RED_AND,
	NVPTX_BUILTIN_BAR_RED_OR, and NVPTX_BUILTIN_BAR_RED_POPC.
	(nvptx_expand_bar_red): New function.
	(nvptx_init_builtins):
	Add DEFs of __builtin_nvptx_bar_red_[and/or/popc].
	(nvptx_expand_builtin): Use nvptx_expand_bar_red to expand
	NVPTX_BUILTIN_BAR_RED_[AND/OR/POPC] cases.

	* config/nvptx/nvptx.md (define_c_enum "unspecv"): Add
	UNSPECV_BARRED_AND, UNSPECV_BARRED_OR, and UNSPECV_BARRED_POPC.
	(BARRED): New int iterator.
	(barred_op,barred_mode,barred_ptxtype): New int attrs.
	(nvptx_barred_<barred_op>): New define_insn.