Re-commit: [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.

(Re-committed with an extra whitespace in SVEInstrFormats.td to trigger rebuild 
of AArch64GenAsmMatcher.inc, since the llvm-clang-x86_64-expensive-checks-win
builder does not seem to rebuild AArch64GenAsmMatcher.inc with the
newly built TableGen due to a missing dependency somewhere (see:
http://lists.llvm.org/pipermail/llvm-dev/2017-December/119555.html))

Reviewers: craig.topper, olista01, rengolin, stoklund

Reviewed By: olista01

Subscribers: javed.absar, llvm-commits

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

llvm-svn: 320711