[MLIR][LLVM] Support inlining of LLVM::AllocaOp.

If inlining into an LLVM function, move allocas of constant size from
the callee's entry block to the new entry block, as this will fold into
the prologue/epilogue code during code generation.

We still allow inlining allocas even if we cannot do this
post-processing (i.e., when we are not inlining into an `LLVM::FuncOp`),
since this is an optimization (and thus not necessary for correctness).

Depends on D141682

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D142436