Fix SILCombine to use OSSA RAUW prepareReplacement
These SILCombine patterns produce new instructions that use the the
original value after it is updated for OSSA. The RAUW utility needs to
copy/borrow the original value before SILCombine can generate the new
replacement. Then SILCombine can pass that replacement back to RAUW to
perform the final replaceAllUsesWith.
1. PointerToAddress and RawPointerToRef
Fixes bugs where RAUW inserts borrows in the wrong place.
2. UncheckedBitwiseCast
The OSSA RAUW helper is incorrect here for producing a new
UncheckedRefCast. Simply don't use it. Since a bitwise cast is an
"Unowned" pointer escape, simply convert the unchecked_bitwise_cast
opcode to unchecked_ref_cast with Unowned forwarding ownership.
3. ConvertFunction
Make sure RAUW is never called with a replacement value that has
different ownership than the original.
