'sqlite3VdbeMemRelease' doesn't alter the value of 'c' but may alter the value of 'pmem->flags' which is what the compiler replaced 'c' with.

from the post by Dr. Richard Hipp "From the -S output, it looks like Clang-11.0.0 is compiling these three lines as if there were written as:"

      sqlite3VdbeMemRelease(pMem);
      pMem->flags = MEM_Str|MEM_Term|(pMem->flags&(MEM_AffMask|MEM_Subtype));