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

Assisted-by: GLM 5.3 Flash

testdata/verify/l64forms_loong64.s

@@ -0,0 +1,51 @@
+// The subtract-immediate fold, the TEQ/TNE trap pseudos, PRELDX, the FP
+// condition branches and the N(PC) branch spellings, against the toolchain.
+#include "textflag.h"
+
+// func SubFold(x int64) int64
+TEXT ·SubFold(SB), NOSPLIT, $0-16
+	MOVV  x+0(FP), R8
+	SUBV  $0, R8
+	SUBV  $4, R9, R10
+	SUBV  $4096, R11
+	SUBV  $-4, R12
+	SUB   $1, R13
+	SUBVU $4, R14
+	SUBV  $1048576, R15
+	MOVV  R8, ret+8(FP)
+	RET
+
+// func Traps(x int64) int64
+TEXT ·Traps(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+	TEQ  $4, R4, R5
+	TEQ  $4, R4
+	TNE  $6, R5, R6
+	MOVV R4, ret+8(FP)
+	RET
+
+// func Prefetch(x int64) int64
+TEXT ·Prefetch(SB), NOSPLIT, $0-16
+	MOVV   x+0(FP), R7
+	PRELDX 0(R7), $0x80001021, $0
+	PRELDX -1(R7), $0x1021, $2
+	MOVV   R7, ret+8(FP)
+	RET
+
+// func BranchForms(x int64) int64
+TEXT ·BranchForms(SB), NOSPLIT, $0-16
+	MOVV x+0(FP), R4
+
+l1:
+	BFPT l1
+	BFPT FCC3, l1
+	BFPF l1
+	JMP  -4(PC)
+	JAL  1(PC)
+	JAL  (R4)
+
+loop:
+	ADDV $1, R4
+	BEQ  R4, R5, loop
+	BNE  R4, l1
+	RET