[AArch64][SVE] Asm: Improve diagnostics further when +sve is not specified

Summary: Patch [4/4] in a series to add parsing of predicates and properly parse SVE ZIP1/ZIP2 instructions. This patch further improves diagnostic messages for when the SVE feature is not specified.

Reviewers: rengolin, fhahn, olista01, echristo, efriedma

Reviewed By: fhahn

Subscribers: sdardis, aemerson, javed.absar, tschuett, llvm-commits, kristof.beyls

Differential Revision: https://reviews.llvm.org/D40363

llvm-svn: 320992