[PATCH] [Tablegen] Fix AsmString parser misreading optional operands.

[Matthew Wahab]

An instruction alias defined with InstAlias and an optional operand in the
middle of the AsmString field, "..${a} <operands>", would get the final
"}" printed in the instruction disassembly. This wouldn't happen if the optional
operand appeared as the last item in the AsmString which is how the current
backends avoided the problem.

There don't appear to be any tests for this part of Tablegen but it passes the
pre-commit tests. Manually tested the change by enabling the generic alias
printer in the ARM backend and checking the output.

REPOSITORY
  rL LLVM

http://reviews.llvm.org/D6529