cmd/internal/obj/riscv: simplify validation and encoding of raw instructions

Use wantImmU/immU rather than handrolling the same code. This also corrects
the validation output - add tests to ensure this is the case.

Change-Id: Id48f459c7c0de09ddde7a10506f66a3a269f325f
Reviewed-on: https://go-review.googlesource.com/c/go/+/702396
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>

Author: 4a6f656c

src/cmd/asm/internal/asm/testdata/riscv64validation.s

@@ -12,6 +12,9 @@ TEXT validation(SB),$0
 	SRLI	$1, X5, F1			// ERROR "expected integer register in rd position but got non-integer register F1"
 	SRLI	$1, F1, X5			// ERROR "expected integer register in rs1 position but got non-integer register F1"
 
+	WORD	$-1				// ERROR "must be in range [0x0, 0xffffffff]"
+	WORD	$0x100000000			// ERROR "must be in range [0x0, 0xffffffff]"
+
 	//
 	// "V" Standard Extension for Vector Operations, Version 1.0
 	//

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

@@ -1419,9 +1419,7 @@ func validateVsetvl(ctxt *obj.Link, ins *instruction) {
 func validateRaw(ctxt *obj.Link, ins *instruction) {
 	// Treat the raw value specially as a 32-bit unsigned integer.
 	// Nobody wants to enter negative machine code.
-	if ins.imm < 0 || 1<<32 <= ins.imm {
-		ctxt.Diag("%v: immediate %d in raw position cannot be larger than 32 bits", ins.as, ins.imm)
-	}
+	wantImmU(ctxt, ins, ins.imm, 32)
 }
 
 // extractBitAndShift extracts the specified bit from the given immediate,
@@ -1706,10 +1704,7 @@ func encodeVsetvl(ins *instruction) uint32 {
 func encodeRawIns(ins *instruction) uint32 {
 	// Treat the raw value specially as a 32-bit unsigned integer.
 	// Nobody wants to enter negative machine code.
-	if ins.imm < 0 || 1<<32 <= ins.imm {
-		panic(fmt.Sprintf("immediate %d cannot fit in 32 bits", ins.imm))
-	}
-	return uint32(ins.imm)
+	return immU(ins.as, ins.imm, 32)
 }
 
 func EncodeBImmediate(imm int64) (int64, error) {