Fold some more [mem] op= val operators. This allows us to things like this

several times in 256.bzip2:

        mov %EAX, DWORD PTR [%ESP + 204]
-       mov %EAX, DWORD PTR [%EAX]
-       or %EAX, 2097152
-       mov %ECX, DWORD PTR [%ESP + 204]
-       mov DWORD PTR [%ECX], %EAX
+       or DWORD PTR [%EAX], 2097152

llvm-svn: 19492