If a hex literal has an 'd' of 'f' in it, such as '0xf' or '0xd', the compiler fails with illegal tree structure.

The other hex literals work.  The issue is probably related to using `f` and `d` in float and double literals.

```
    public void testHexCollisionDouble() {
        assertEquals(13, exec("return 0xd"));
    }

    public void testHexCollisionFloat() {
        assertEquals(15, exec("return 0xf"));
    }

    public void testHex() throws Exception {
        for (int i = 0; i <= 15; i++) {
            String hex = Integer.toHexString(i);
            if ("d".equals(hex) || "f".equals(hex)) {
                continue;
            }
            assertEquals(i, exec("return 0x" + hex));
            assertEquals(i, exec("return 0x" + hex.toUpperCase(Locale.ROOT)));
            assertEquals(i, exec("return 0X" + hex));
            assertEquals(i, exec("return 0X" + hex.toUpperCase(Locale.ROOT)));
        }
    }
```
modules/lang-painless/src/main/java/org/elasticsearch/painless/phase/DefaultSemanticAnalysisPhase.java
* Visits a conditional expression.
     * Checks: type validation
     */
    @Override
    public void visitConditional(EConditional userConditionalNode, SemanticScope semanticScope) {
        if (semanticScope.getCondition(userConditionalNode, Write.class)) {
            throw userConditionalNode.createError(
                new IllegalArgumentException("invalid assignment: cannot assign a value to conditional operation [?:]")
            );
        }
        if (semanticScope.getCondition(userConditionalNode, Read.class) == false) {
            throw userConditionalNode.createError(
                new IllegalArgumentException("not a statement: result not used from conditional operation [?:]")
            );
        }
        AExpression userConditionNode = userConditionalNode.getConditionNode();
        semanticScope.setCondition(userConditionNode, Read.class);
        semanticScope.putDecoration(userConditionNode, new TargetType(boolean.class));
        checkedVisit(userConditionNode, semanticScope);
        decorateWithCast(userConditionNode, semanticScope);
        AExpression userTrueNode = userConditionalNode.getTrueNode();
        semanticScope.setCondition(userTrueNode, Read.class);
        semanticScope.copyDecoration(userConditionalNode, userTrueNode, TargetType.class);
        semanticScope.replicateCondition(userConditionalNode, userTrueNode, Explicit.class);
        semanticScope.replicateCondition(userConditionalNode, userTrueNode, Internal.class);
        checkedVisit(userTrueNode, semanticScope);
        Class<?> leftValueType = semanticScope.getDecoration(userTrueNode, ValueType.class).valueType();
        AExpression userFalseNode = userConditionalNode.getFalseNode();
        semanticScope.setCondition(userFalseNode, Read.class);
        semanticScope.copyDecoration(userConditionalNode, userFalseNode, TargetType.class);