feat(riscv): add MOV pseudo-instruction and RVC compressed encoding

Assisted-by: DeepSeek V4 Pro

testdata/lrsc_cvt_riscv64.s

@@ -4,34 +4,33 @@
 #include "textflag.h"
 
 // func casLoop(ptr *int64, old, new int64) bool
-// Compare-and-swap using LR/SC.
 TEXT ·casLoop(SB), NOSPLIT, $0-32
 cas_retry:
-	LD a+0(FP), A0
-	LRD (A0), A1
-	LD b+8(FP), A2
-	BNE A1, A2, cas_fail
-	LD c+16(FP), A3
-	SCD A3, (A0), A4
-	BNE A4, X0, cas_retry
-	ADDI X0, $1, A5
-	SD A5, ret+24(FP)
-	RET
+        MOV a+0(FP), X10
+        LRD (X10), X11
+        MOV b+8(FP), X12
+        BNE X11, X12, cas_fail
+        MOV c+16(FP), X13
+        SCD X13, (X10), X14
+        BNE X14, X0, cas_retry
+        ADDI X0, $1, X15
+        MOV X15, ret+24(FP)
+        RET
 cas_fail:
-	SD X0, ret+24(FP)
-	RET
+        MOV X0, ret+24(FP)
+        RET
 
 // func intToFloat(x int64) float64
 TEXT ·intToFloat(SB), NOSPLIT, $0-16
-	LD a+0(FP), A0
-	FCVTDL A0, FA0
-	FSD FA0, ret+8(FP)
-	RET
+        MOV a+0(FP), X10
+        FCVTDL X10, F10
+        FSD F10, ret+8(FP)
+        RET
 
 // func compare(a, b float64) bool
 TEXT ·compare(SB), NOSPLIT, $0-24
-	FLD a+0(FP), FA0
-	FLD b+8(FP), FA1
-	FLTD FA0, FA1, A0
-	SD A0, ret+16(FP)
-	RET
+        FLD a+0(FP), F10
+        FLD b+8(FP), F11
+        FLTD F10, F11, X10
+        MOV X10, ret+16(FP)
+        RET