df.eval fails for Unary operation involving float32 column
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd

df = pd.DataFrame({
    'Foo64': [1.2, 2.4, 7.24],
})

df['Foo32'] = df['Foo64'].astype('float32')

df.eval('-1 + Foo64')   # Works
df.eval('-1 + Foo32')   # Throws Exception
```

```python traceback
Traceback (most recent call last):
  File "C:\Users\vmuriart\Desktop\bug_eval.py", line 10, in <module>
    df.eval('-1 + Foo32')  # Throws Exception
  File "C:\Anaconda\lib\site-packages\pandas\core\frame.py", line 2186, in eval
    return _eval(expr, inplace=inplace, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\eval.py", line 262, in eval
    truediv=truediv)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 727, in __init__
    self.terms = self.parse()
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 744, in parse
    return self._visitor.visit(self.expr)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit
    return visitor(node, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 319, in visit_Module
    return self.visit(expr, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit
    return visitor(node, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 322, in visit_Expr
    return self.visit(node.value, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 313, in visit
    return visitor(node, **kwargs)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 417, in visit_BinOp
    left, right = self._maybe_downcast_constants(left, right)
  File "C:\Anaconda\lib\site-packages\pandas\core\computation\expr.py", line 365, in _maybe_downcast_constants
    name = self.env.add_tmp(np.float32(left.value))
AttributeError: 'UnaryOp' object has no attribute 'value'

```
#### Problem description

`df.eval(...)` operations fail if one of the columns involved is of type `float32` and some `unary` operations are involved. In the example above `df.eval('-1 + Foo32')` failed, but `df.eval('-Foo32')` will succeed, and `df.eval('-1*Foo32')` will also fail.

Originally I tested fixing this by just adding `self.value = operand.value` to the `UnaryOp` [object](https://github.com/pandas-dev/pandas/blob/master/pandas/core/computation/ops.py#L497), before I realized that this issue only affects `float32` objects. I haven't looked to see where the code branches off to cause this misbehavior. 

This was tested and reproduced on both `0.20.3` and `0.22.0`

#### Expected Output

No exception thrown.

#### Output of ``pd.show_versions()``

<details>


INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 79 Stepping 1, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.22.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.12.1
scipy: 0.19.1
pyarrow: 0.7.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
