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
5dedf911de