evalv3: "field not allowed" regression inside an if comprehension

[mvdan]

Reduced from https://github.com/tmm1/taxes.cue/blob/c7a9fefa2fa115b5d06fb952636a475823ff14a9/return.cue#L215, which is correctly defined in the #Form1040 schema.

As of b51914f:

env CUE_EXPERIMENT=evalv3=0
exec cue vet -c=false in.cue

env CUE_EXPERIMENT=evalv3=1
exec cue vet -c=false in.cue

-- in.cue --
#Schema: {
	schemaField: number
}

out1: {
	out2: #Schema & {
		if false {
			schemaField: 3
		}
	}
}

I would expect this testscript to pass - the old and new evaluator should agree that the above is valid CUE. However:

> env CUE_EXPERIMENT=evalv3=0
> exec cue vet -c=false in.cue
> env CUE_EXPERIMENT=evalv3=1
> exec cue vet -c=false in.cue
[stderr]
out1.out2.schemaField: field not allowed:
    ./in.cue:8:17
    ./in.cue:2:2
    ./in.cue:8:4
[exit status 1]
FAIL: f1040.cue:5: unexpected command failure

The error seems wrong; we are only closing one struct here, #Schema, and it does define and allow the schemaField field.

Note that the evalv3 regression goes away if I swap if false for if true. Similarly, if I swap schemaField: 3 for an embedding like { schemaField: 3 }, the error also goes away. Both seem supporting evidence for my reasoning above.