[ConstantFold] Check for uniform value before reinterpret load

The reinterpret load code will convert undef values into zero.
Check the uniform value case before it to produce a better result
for all-undef initializers.

However, the uniform value handling will return the uniform value
even if the access is out of bounds, while the reinterpret load
code will return undef. Add an explicit check to retain the
previous result in this case.