[mlir][LLVMIR] Do not cache llvm::Constant into instMap

Constants in MLIR are not globally unique, unlike that in LLVM IR.
Therefore, reusing previous-translated constants might cause the user
operations not being dominated by the constant (because the
previous-translated ones can be placed in arbitrary place)

This indeed misses some opportunities where we actually can reuse a
previous-translated constants, but verbosity is not our first priority
here.

Differential Revision: https://reviews.llvm.org/D124404