[mlir] Erase or clear blocks through ConversionPatternRewriter when applicable

Multiple places in the code base were erasing Blocks or operations in them
using in-place modifications (`Block::erase` or `Block::clear`) unknown to
ConversionPatternRewriter. These operations could not be undone if the pattern
failed and could lead to inconsistent in-memory state of the IR with dangling
pointers. Use `ConversionPatternRewriter::eraseOp` and `::eraseBlock` instead.

Differential Revision: https://reviews.llvm.org/D80136