[AArch64][SVE] Enable DiagnosticPredicates for SVE LD1 instructions.

This patch extends the PredicateMethod of AsmOperands used in SVE's 
LD1 instructions with a DiagnosticPredicate. This makes them 'context
sensitive' to the operand that has been parsed and tells the user to
use the right register (with expected shift/extend), rather than telling 
the immediate is out of range when it actually parsed a register.

Patch [2/2] in a series to improve assembler diagnostics for SVE:
-  Patch [1/2]: https://reviews.llvm.org/D45879
-  Patch [2/2]: https://reviews.llvm.org/D45880

Reviewers: olista01, stoklund, craig.topper, mcrosier, rengolin, echristo, fhahn, SjoerdMeijer, evandro, javed.absar

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D45880

llvm-svn: 330934