[mlir][MemRefToLLVM] Fix crashes with unconvertable memory spaces (#70694)

Fixes #70160

The issue is resolved by:
1. Changing the call to address space conversion to use the correct
return type, preventing the code from moving past the if and into the
crashing optional dereference.
2. Adding handling to the AllocLikeOp rewriter for the case where the
underlying buffer allocation fails.