LLVM IR lowering: support 'dim' operation.

Add support for translating 'dim' opreation on MemRefs to LLVM IR.  For a
static size, this operation merely defines an LLVM IR constant value that may
not appear in the output IR if not used (and had not been removed before by
DCE).  For a dynamic size, this operation is translated into an access to the
MemRef descriptor that contains the dynamic size.

PiperOrigin-RevId: 223160774