[AMDGPU] Don't rely on !eq comparing int with bits<5>. NFC. (#113279)
Tweak VOP2eInst_Base so that it does not rely on !eq comparing an int
value (-1) with a bits<5> value. This is to avoid a change in behaviour
when #112904 lands, which is a bug fix which has the side effect of
implicitly casting template arguments to the declared template parameter
type.