AsmMatcher custom operand parser failure enhancements.

AsmMatcher custom operand parser failure enhancements.

Teach the AsmMatcher handling to distinguish between an error custom-parsing
an operand and a failure to match. The former should propogate the error
upwards, while the latter should continue attempting to parse with
alternative matchers.

Update the ARM asm parser accordingly.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@125426 91177308-0d34-0410-b5e6-96231b3b80d8