New opcodes to deal with 8-bit immediate sign extended to opsize

New opcodes to deal with 8-bit immediates which are then sign-extended
to the operand size.  These allow us to warn appropriately.
Not sure I'm using these in all the proper places; need audit of all
uses of the \14..\17 opcodes.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
diff --git a/disasm.c b/disasm.c
index f3d4d2a..46e3cf2 100644
--- a/disasm.c
+++ b/disasm.c
@@ -482,6 +482,7 @@
 	}
 
 	case4(014):
+	case4(0274):
             opx->offset = (int8_t)*data++;
             opx->segment |= SEG_SIGNED;
 	    break;