Use correct sort for x in #testPowerAddiSymbolic

In this line:

``` self assert: (instr fieldValue: 'd') equals: 'x'

the `instr fieldValue: 'd'` is a Z3Int, so the #= down in assert:equals: coerces the 'x' to a Z3Int.  So now we have two `x` symbols: a BV16 `x` and an Int `x`, which are not necessarily equal.  The correct thing to do is to start from the same BV `x`, and compare the Int interpretation of `d` to the signed Int interpretation of `x`.

src/ArchC-Core-Tests/AcInstrDecodeTest.class.st

@@ -65,7 +65,7 @@ AcInstrDecodeTest >> testPowerAddiSymbolic [
 	self assert: instr name equals: 'addi'.
 	self assert: (instr fieldValue: 'rt') equals: 0.
 	self assert: (instr fieldValue: 'ra') equals: 0.
-	self assert: (instr fieldValue: 'd') equals: 'x'.
+	self assert: (instr fieldValue: 'd') equals: ('x'///16) toSignedInt .
 
 ]