feat(riscv64,loong64): PCALIGN, branch relaxation and operand shapes

Assisted-by: GLM 5.3 Flash

testdata/verify/pcalign_riscv64.s

@@ -0,0 +1,35 @@
+// PCALIGN padding on riscv64: 4-byte NOPs with a 2-byte compressed NOP when
+// the pad is 2 mod 4, exactly as the toolchain lays the bytes down.
+#include "textflag.h"
+
+// func Pad8(x int64) int64
+TEXT ·Pad8(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $8
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func Pad16(x int64) int64
+TEXT ·Pad16(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $16
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func Pad32(x int64) int64
+TEXT ·Pad32(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $32
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET
+
+// func PadAfterOdd(x int64) int64
+TEXT ·PadAfterOdd(SB), NOSPLIT, $0-16
+	MOV     x+0(FP), X5
+	PCALIGN $8
+	ADD     $1, X5
+	MOV     X5, ret+8(FP)
+	RET