[mlir][LLVMIR] Allow !llvm.ptr<ptr> operands in atomicrmw xchg op.

Previously, llvm.atomicrmw only allowed operands that are pointers to
LLVM floats or integers. However, according to the LLVM IR Language
Reference, that op allows pointer to pointer operands in its `xchg`
mode. This patch allows those operands also in MLIR's LLVM dialect and
adapts the tests accordingly.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D153747