[mlir] Swap use of to_vector() with lookupValues() in LLVMIRIntrinsicGen

Summary:
llvm::to_vector() accepts a Range value and not the pair of arguments
we are currently passing. Also we probably want the lowered LLVM
values in the vector, while operand_begin()/operand_end() on MLIR ops
returns MLIR types. lookupValues() seems the correct way to collect
such values.

Reviewers: rriddle, andydavis1, antiagainst, nicolasvasilache, ftynse

Subscribers: jdoerfert, mehdi_amini, jpienaar, burmako, shauheen, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73137