Bug: In-line extract not accepted by analog and atomic grammar

The following code is accepted by analog grammar:
```
r = qreg[2]
q0 = r[0]
H_single = 0.5 %* %X %+ 0.5 %* %Z
evolve(H_single, 1.0, q0)
```
Note that `evolve` is applied on the target `q0`. However, the following code raises an error:
```
r = qreg[2]
H_single = 0.5 %* %X %+ 0.5 %* %Z
evolve(H_single, 1.0, r[0])
```
This is because the register `r[0]` is defined in-line by the program.