nvptx: Add support for PTX's cnot instruction.

nvptx: Add support for PTX's cnot instruction.

This is a simple patch, now that the nvptx backend has transitioned
to STORE_FLAG_VALUE=1, that adds support for NVidia's cnot instruction,
that implements C/C++ style logical negation.

Previously, the simple function:

int foo(int x) { return !x; }

on nvptx-none with -O2 would generate:

	mov.u32 %r24, %ar0;
	setp.eq.u32     %r28, %r24, 0;
	selp.u32        %value, 1, 0, %r28;

with this patch, GCC now generates:

	mov.u32 %r24, %ar0;
	cnot.b32        %value, %r24;

2022-01-07  Roger Sayle  <roger@nextmovesoftware.com>

gcc/ChangeLog
	* config/nvptx/nvptx.md (*cnot<mode>2): New define_insn.

gcc/testsuite/ChangeLog
	* gcc.target/nvptx/cnot-1.c: New test case.