[TableGen] AsmMatcher: Fix bug with reported diagnostic for operand.

[TableGen] AsmMatcher: Fix bug with reported diagnostic for operand.

Summary:
The generated diagnostic by the AsmMatcher isn't always applicable to the AsmOperand.

This is because the code will only update the diagnostic if it is more specific than the previous diagnostic. However, when having validated operands and 'moved on' to a next operand (for some instruction/alias for which all previous operands are valid), if the diagnostic is InvalidOperand, than that should be set as the diagnostic, not the more specific message about a previous operand for some other instruction/alias candidate.

Reviewers: craig.topper, olista01, rengolin, stoklund

Reviewed By: olista01

Subscribers: javed.absar, llvm-commits

Differential Revision: https://reviews.llvm.org/D40011

llvm-svn: 318759