[AArch64][SVE] Asm: Fix predicate pattern diagnostics.

This patch uses the DiagnosticPredicate for SVE predicate patterns
to improve their diagnostics, now giving a 'invalid operand' diagnostic
if the type is not an immediate or one of the expected pattern
labels.

Reviewers: samparker, SjoerdMeijer, javed.absar, fhahn

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D48220

llvm-svn: 334983