[AsmParser] Match mandatory operands following optional operands.

[AsmParser] Match mandatory operands following optional operands.

Currently, the asm parser stops matching instruction operands as soon as the first optional operand is encountered. This leads to the need for custom checks on missing mandatory operands that come after optional operands.

The patch changes the parser to always match all optional and mandatory instruction operands, thus making the custom checks unnecessary. This is particularly useful for the AMDGPU backend where we have numerous optional instruction modifiers.

Reviewed By: dp

Differential Revision: https://reviews.llvm.org/D137549