Add repeat support for memory addresses.

Rationale:
This enables exhaustive testing of instructions
that use memory addresses. First use case of
the generics is x86.

Bug: 18380245
Bug: 18380559
Bug: 18380348

Test: assembler_x86[_64]_test

Change-Id: Ib0ad6fa65477b0c6fc04642ff980a4b9543d16d5