[mlir] DialectConversion: avoid double-free when rolling back op creation

Dialect conversion infrastructure may roll back op creation by erasing the
operations in the reverse order of their creation. While this guarantees uses
of values will be deleted before their definitions, this does not guarantee
that a parent operation will not be deleted before its child. (This may happen
in case of block inlining or if child operations, such as terminators, are
created in the parent's `build` function before the parent itself.) Handle the
parent/child relationship between ops by removing all child ops from the blocks
before erasing the parent. The child ops remain live, detached from a block,
and will be safely destroyed in their turn, which may come later than that of
the parent.

Differential Revision: https://reviews.llvm.org/D80134