[AArch64] ldr= pseudo-instruction silently ignored if register invalid

The AArch64 assembler was silently ignoring instructions like this:
  ldr foo, =bar

AArch64AsmParser::parseOperand was returning true as the parse failed, but was
not calling AArch64AsmParser::Error to report this to the user, so the
instruction was ignored without printing an error message.

Differential Revision: http://reviews.llvm.org/D14651

llvm-svn: 253193