[mlir][DialectConversion] Emit an error if an operation marked as erased has...

[mlir][DialectConversion] Emit an error if an operation marked as erased has live users after conversion

Up until now, there has been an implicit agreement that when an operation is marked as
"erased" all uses of that operation's results are guaranteed to be removed during conversion. How this works in practice is that there is either an assert/crash/asan failure/etc. This revision adds support for properly detecting when an erased operation has dangling users, emits and error and fails the conversion.

Differential Revision: https://reviews.llvm.org/D82830