[mlir][LLVMIR] Memorize compatible LLVM types

This patch memorize compatible LLVM types in `LLVM::isCompatibleType` in
order to avoid redundant works.

This is especially useful when the size of program is big and there are
multiple occurrences of some deeply nested LLVM struct types, in which
case we can gain quite some speedups with this patch.

Differential Revision: https://reviews.llvm.org/D127918