[AArch64] Attempt to parse more operands as expressions
This tries to make use of evaluateAsRelocatable in AArch64AsmParser::classifySymbolRef
to parse more complex expressions as relocatable operands. It is hopefully better than
the existing code which only handles Symbol +- Constant.
This allows us to parse more complex adr/adrp, mov, ldr/str and add operands. It also
loosens the requirements on parsing addends in ld/st and mov's and adds a number of
tests.
Differential Revision: https://reviews.llvm.org/D51792
llvm-svn: 342455