[AsmParser] Rework logic around "region argument parsing"

The asm parser had a notional distinction between parsing an
operand (like "%foo" or "%4#3") and parsing a region argument
(which isn't supposed to allow a result number like #3).

Unfortunately the implementation has two problems:

1) It didn't actually check for the result number and reject
   it.  parseRegionArgument and parseOperand were identical.
2) It had a lot of machinery built up around it that paralleled
   operand parsing.  This also was functionally identical, but
   also had some subtle differences (e.g. the parseOptional
   stuff had a different result type).

I thought about just removing all of this, but decided that the
missing error checking was important, so I reimplemented it with
a `allowResultNumber` flag on parseOperand.  This keeps the
codepaths unified and adds the missing error checks.

Differential Revision: https://reviews.llvm.org/D124470