[mlir] Test CallOp STD->LLVM conversion.

This exercises the corner case that was fixed in
https://reviews.llvm.org/rG8979a9cdf226066196f1710903d13492e6929563.

The bug can be reproduced when there is a @callee with a custom type argument and @caller has a producer of this argument passed to the @callee.

Example:
func @callee(!test.test_type) -> i32
func @caller() -> i32 {
  %arg = "test.type_producer"() : () -> !test.test_type
  %out = call @callee(%arg) : (!test.test_type) -> i32
  return %out : i32
}

Even though there is a type conversion for !test.test_type, the output IR (before the fix) contained a DialectCastOp:

module {
  llvm.func @callee(!llvm.ptr<i8>) -> !llvm.i32
  llvm.func @caller() -> !llvm.i32 {
    %0 = llvm.mlir.null : !llvm.ptr<i8>
    %1 = llvm.mlir.cast %0 : !llvm.ptr<i8> to !test.test_type
    %2 = llvm.call @callee(%1) : (!test.test_type) -> !llvm.i32
    llvm.return %2 : !llvm.i32
  }
}

instead of

module {
  llvm.func @callee(!llvm.ptr<i8>) -> !llvm.i32
  llvm.func @caller() -> !llvm.i32 {
    %0 = llvm.mlir.null : !llvm.ptr<i8>
    %1 = llvm.call @callee(%0) : (!llvm.ptr<i8>) -> !llvm.i32
    llvm.return %1 : !llvm.i32
  }
}

Differential Revision: https://reviews.llvm.org/D85914