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

Assisted-by: GLM 5.3 Flash

testdata/verify/pcalign_loong64.s

@@ -0,0 +1,33 @@
+// PCALIGN padding on loong64: andi $0, $0, 0 (the architecture's NOP), plus
+// the automatic loop-head alignment to a 16-byte boundary.
+#include "textflag.h"
+
+// func Pad16(x int64) int64
+TEXT ·Pad16(SB), NOSPLIT, $0-16
+	MOVV    x+0(FP), R4
+	PCALIGN $16
+	ADDV    $1, R4
+	MOVV    R4, ret+8(FP)
+	RET
+
+// func Pad32(x int64) int64
+TEXT ·Pad32(SB), NOSPLIT, $0-16
+	MOVV    x+0(FP), R4
+	PCALIGN $32
+	ADDV    $1, R4
+	MOVV    R4, ret+8(FP)
+	RET
+
+// func LoopAlign(x int64) int64
+TEXT ·LoopAlign(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+	MOVV $10, R5
+
+loop:
+	BEQ  R4, R5, done
+	ADDV $1, R4
+	JMP  loop
+
+done:
+	MOVV R4, ret+8(FP)
+	RET