[CodeGenPrepare] Don't create dead instructions in addrmode sinking

[CodeGenPrepare] Don't create dead instructions in addrmode sinking

When we fail to sink an instruction, we must make sure not to modify
the function; otherwise, we end up in an infinite loop because
CodeGenPrepare iterates until it doesn't make any changes.

Fixes https://bugs.llvm.org/show_bug.cgi?id=33608 .

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@307866 91177308-0d34-0410-b5e6-96231b3b80d8