CoroFrame: Fix missing bitcast for some frame merges

The code to adjust types when merging fields on the coroutine frame
accidentally used `GetElementPtrInst::getResultElementType()` which does
the return the type the resulting pointer points to (so misses a pointer
type). This worked most of the time by accident because the type nearly
never matched, but failed to add a necessary bitcast for from `i8**` to
`i8*`.

Differential Revision: https://reviews.llvm.org/D142385