operand: allow signed immediates to be matched by IsIMMX (#182)

Many of the instruction functions correctly match the size of immediate values, but they only accept unsigned immediates. This PR fixes the operand check functions for intermediate types to also accept the signed variants.

Fixes #181

operand/checks.go

@@ -35,26 +35,30 @@ func IsIMM2U(op Op) bool {
 
 // IsIMM8 returns true is op is an 8-bit immediate.
 func IsIMM8(op Op) bool {
-	_, ok := op.(U8)
-	return ok
+	_, uok := op.(U8)
+	_, iok := op.(I8)
+	return uok || iok
 }
 
 // IsIMM16 returns true is op is a 16-bit immediate.
 func IsIMM16(op Op) bool {
-	_, ok := op.(U16)
-	return ok
+	_, uok := op.(U16)
+	_, iok := op.(I16)
+	return uok || iok
 }
 
 // IsIMM32 returns true is op is a 32-bit immediate.
 func IsIMM32(op Op) bool {
-	_, ok := op.(U32)
-	return ok
+	_, uok := op.(U32)
+	_, iok := op.(I32)
+	return uok || iok
 }
 
 // IsIMM64 returns true is op is a 64-bit immediate.
 func IsIMM64(op Op) bool {
-	_, ok := op.(U64)
-	return ok
+	_, uok := op.(U64)
+	_, iok := op.(I64)
+	return uok || iok
 }
 
 // IsAL returns true if op is the AL register.

operand/checks_test.go

@@ -36,6 +36,12 @@ func TestChecks(t *testing.T) {
 
 		{IsIMM64, Imm((1 << 64) - 1), true},
 
+		// Signed Immediates
+		{IsIMM8, I8(-1), true},
+		{IsIMM16, I16(-1), true},
+		{IsIMM32, I32(-1), true},
+		{IsIMM64, I64(-1), true},
+
 		// Specific registers
 		{IsAL, reg.AL, true},
 		{IsAL, reg.CL, false},