IfConversion: Add assertions that both sides of a diamond don't pred-clobber.

One side of a diamond may end with a predicate clobbering instruction.
That side of the diamond has to be if-converted second. Both sides can't
clobber the predicate or the ifconversion is invalid. This is checked
elsewhere, but add an assert as a safety check. NFC

llvm-svn: 280518