Fix FxPick code emission

- Fixed: Integer constants passed to pick() need to manually generate load
  instructions, since FxConstant::Emit() will just return a constant
  register with its value.
- Fixed: VMFunctionBuilder::RegAvailability::Reuse() didn't actually
  calculate a proper mask. Also added another assert to this function.

src/thingdef/thingdef_expression.cpp

@@ -2203,6 +2203,13 @@ ExpEmit FxPick::Emit(VMFunctionBuilder *build)
 	for (unsigned i = 0; i < choices.Size(); ++i)
 	{
 		build->BackpatchToHere(jumptable + i);
+		if (choices[i]->isConstant())
+		{
+			int val = static_cast<FxConstant *>(choices[i])->GetValue().GetInt();
+			build->EmitLoadInt(resultreg.RegNum, val);
+		}
+		else
+		{
 			ExpEmit casereg = choices[i]->Emit(build);
 			if (casereg.RegNum != resultreg.RegNum)
 			{ // The result of the case is in a different register from what
@@ -2214,6 +2221,7 @@ ExpEmit FxPick::Emit(VMFunctionBuilder *build)
 			}
 			// Free this register so the remaining cases can use it.
 			casereg.Free(build);
+		}
 		// All but the final case needs a jump to the end of the expression's code.
 		if (i + 1 < choices.Size())
 		{

src/zscript/vmbuilder.cpp

@@ -423,8 +423,9 @@ void VMFunctionBuilder::RegAvailability::Return(int reg, int count)
 bool VMFunctionBuilder::RegAvailability::Reuse(int reg)
 {
 	assert(reg >= 0 && reg <= 255);
+	assert(reg < MostUsed && "Attempt to reuse a register that was never used");
 
-	VM_UWORD mask = reg & 31;
+	VM_UWORD mask = 1 << (reg & 31);
 	int word = reg / 32;
 
 	if (Used[word] & mask)