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
new IllegalArgumentException(
                                        "field [" + prefixStaticType.getStaticCanonicalTypeName() + ", " + index + "] not found"
                                    )
                                );
                            }
                        }
                    } else {
                        if (write && Modifier.isFinal(field.javaField().getModifiers())) {
                            throw userDotNode.createError(
                                new IllegalArgumentException(
                                    "invalid assignment: cannot assign a value to read-only field [" + field.javaField().getName() + "]"
                                )
                            );
                        }
                        semanticScope.putDecoration(userDotNode, new StandardPainlessField(field));
                        valueType = field.typeParameter();
                    }
                }
                semanticScope.putDecoration(userDotNode, new ValueType(valueType));
                if (userDotNode.isNullSafe()) {
                    if (write) {
                        throw userDotNode.createError(
                            new IllegalArgumentException("invalid assignment: cannot assign a value to a null safe operation [?.]")
                        );
                    }
                    if (valueType.isPrimitive()) {
                        throw new IllegalArgumentException("Result of null safe operator must be nullable");
                    }
                }
            }