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
constant = Float.parseFloat(decimal.substring(0, decimal.length() - 1));
                valueType = float.class;
            } catch (NumberFormatException exception) {
                throw userDecimalNode.createError(new IllegalArgumentException("Invalid float constant [" + decimal + "]."));
            }
        } else {
            String toParse = decimal;
            if (toParse.endsWith("d") || decimal.endsWith("D")) {
                toParse = toParse.substring(0, decimal.length() - 1);
            }
            try {
                constant = Double.parseDouble(toParse);
                valueType = double.class;
            } catch (NumberFormatException exception) {
                throw userDecimalNode.createError(new IllegalArgumentException("Invalid double constant [" + decimal + "]."));
            }
        }
        semanticScope.putDecoration(userDecimalNode, new ValueType(valueType));
        semanticScope.putDecoration(userDecimalNode, new StandardConstant(constant));
    }
    /**
     * Visits a string constant expression.
     * Checks: type validation
     */
    @Override
    public void visitString(EString userStringNode, SemanticScope semanticScope) {
        String string = userStringNode.getString();
        if (semanticScope.getCondition(userStringNode, Write.class)) {
            throw userStringNode.createError(
                new IllegalArgumentException("invalid assignment: cannot assign a value to string constant [" + string + "]")