[mlir][LLVMIR] Do not cache Instruction generated on-the-fly

More specifically, the llvm::Instruction generated by
llvm::ConstantExpr::getAsInstruction. Such Instruction will be deleted
right away, but it's possible that when getAsInstruction is called
again, it will create a new Instruction that has the same address with
the one we just deleted. Thus, we shouldn't keep it in the `instMap` to
avoid a conflicting index that triggers an assertion in
processInstruction.

Differential Revision: https://reviews.llvm.org/D124402