[OpaquePtrs] Remove some uses of type-less CreateGEP() (NFC)

This removes some (but not all) uses of type-less CreateGEP()
and CreateInBoundsGEP() APIs, which are incompatible with opaque
pointers.

There are a still a number of tricky uses left, as well as many
more variation APIs for CreateGEP.

GitOrigin-RevId: 42eb658f656c43ee63e25222f32abbbbcb86ad7b
1 file changed