cmd/internal/obj/riscv: add NOT pseudo-instruction

Add a NOT pseudo-instruction that translates to XORI $-1.

Change-Id: I2be4cfe2939e988cd7f8d30260b704701d78475f
Reviewed-on: https://go-review.googlesource.com/c/go/+/221688
Reviewed-by: Cherry Zhang <cherryyz@google.com>

src/cmd/asm/internal/asm/testdata/riscvenc.s

@@ -305,10 +305,14 @@ start:
 	MOVD	F0, 4(X5)				// 27b20200
 	MOVD	F0, F1					// d3000022
 
+	// NOT pseudo-instruction
+	NOT	X5					// 93c2f2ff
+	NOT	X5, X6					// 13c3f2ff
+
 	// These jumps can get printed as jumps to 2 because they go to the
 	// second instruction in the function (the first instruction is an
 	// invisible stack pointer adjustment).
-	JMP	start		// JMP	2		// 6ff09fc6
+	JMP	start		// JMP	2		// 6ff01fc6
 	JMP	(X5)					// 67800200
 	JMP	4(X5)					// 67804200
 

src/cmd/internal/obj/riscv/anames.go

@@ -239,6 +239,7 @@ var Anames = []string{
 	"MOVHU",
 	"MOVW",
 	"MOVWU",
+	"NOT",
 	"SEQZ",
 	"SNEZ",
 	"LAST",

src/cmd/internal/obj/riscv/cpu.go

@@ -589,6 +589,7 @@ const (
 	AMOVHU
 	AMOVW
 	AMOVWU
+	ANOT
 	ASEQZ
 	ASNEZ
 

src/cmd/internal/obj/riscv/obj.go

@@ -1849,6 +1849,15 @@ func instructionsForProg(p *obj.Prog) []*instruction {
 		ins.rs1 = uint32(p.From.Reg)
 		ins.rs2 = REG_F0
 
+	case ANOT:
+		// NOT rs, rd -> XORI $-1, rs, rd
+		ins.as = AXORI
+		ins.rs1, ins.rs2 = uint32(p.From.Reg), obj.REG_NONE
+		if ins.rd == obj.REG_NONE {
+			ins.rd = ins.rs1
+		}
+		ins.imm = -1
+
 	case ASEQZ:
 		// SEQZ rs, rd -> SLTIU $1, rs, rd
 		ins.as = ASLTIU