Fix logic error in test guarding prototype MIR borrowck code.
Fix logic error in test guarding prototype MIR borrowck code.

(This crept in during the shift from a transform to a query; I didn't
notice because my muscle memory was still always passing `-Z
mir-borrowck`, while my test cases *also* had the
`#[rustc_mir_borrowck]` attribute attached to them.)

aidancully/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,aidancully/rust,aidancully/rust