[mlir] Stop allowing LLVMType Int arguments for GPULaunchFuncOp.

Conversion to LLVM becomes confusing and incorrect if someone tries to lower
STD -> LLVM and only then GPULaunchFuncOp to LLVM separately. Although it is
technically allowed now, it works incorrectly because of the argument
promotion. The correct way to use this conversion pattern is to add to the
STD->LLVM patterns before running the pass.

Differential Revision: https://reviews.llvm.org/D88147